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Climatic and Environmental Challenges: Learning from the Horn of Africa

 | 
David Ambrosetti
, 
Jean-Renaud Boisserie
, 
Deresse Ayenachew
, 
et al.

Environmental Changes and Their Impacts on Life: A Paleontological Approach

Jean-Renaud Boisserie

Entrées d'index

Géographique :

Omo (vallée)

Texte intégral

The “Fossil Earth”

1We – meaning the whole living humankind – are facing a fast climatic change that is a harbinger of forthcoming dismal days (IPCC, 2014). Yet, we have a hard time at agreeing on what should be done to limit the effects of this change, as reflected by the resounding failure of the 2009 Copenhagen summit on climate change. Over the years, climatologists have refined their prospective scenarios based on ever more elaborate virtual climatic modellings, and they have convinced most of the scientific community. However, the conclusions of the IPCC report continue to trigger disinterest, suspicion, or even hostility from many policy makers, economic stakeholders, media and ordinary people, unwilling to change their present way of life and society.

  • 1 More pragmatically, Pratchett & Baxter’s multiverse (2012), by providing millions of human-empty wo (...)

2To overcome this resistance, the wonderful science fiction author Terry Pratchett offered a solution. In The Long Earth, Pratchett & Baxter (2012) imagined that our Earth is only one iteration within an apparently infinite succession of parallel earths displaying slight climatic differences between each other, and that a small side step would be enough to travel from one iteration to another. By studying these parallel earths, scientists could work on life-size earth models, dropping most of the built-in uncertainties of their virtual models. Even better, “climate change skeptics” could see for themselves the effects of climate variations and finally understand what on-going changes mean for the whole humankind, themselves included1.

3Frivolous fancy? Yet I believe that, given the magnitude of our growing problems, we need all our assets, and the ability to envision the impossible is certainly not the least of them. Besides, I have good news: the Long Earth actually exists, and we can explore it!

4Of course, it is not exactly the horizontal quantum continuum described by Pratchett & Baxter (2012); instead our long earth is formed by another type of parallel realities, vertically stacked within a temporal continuum of ca. 4.54 billion years, which could be called the “Fossil Earth.” Indeed, the deep past of our planet2 is a continuous sequence of extinct worlds that differ in their climatic conditions. Given their antiquity, the fossil witnesses of these worlds are unavoidably fragmentary and distorted. Yet, thanks to them and to hard work, we can study the main features and events of Fossil Earth worlds.

5We can consider them as many “in vivo experiments” performed at global scale during thousands, hundred thousands, and millions of years. These experiments have an educational dimension demonstrating the effects of ancient events that can be compared with today’s on-going changes, as well as a practical dimension: past data allows us to reach a deeper understanding of what is going on. This understanding is of course based on observing the incipient effects of on-going changes; but in the best cases, these observations started only a few decades ago, i.e. a ridiculously brief period of time considering the antiquity, complexity, and size of Earth. This results in a strong degree of uncertainty in extrapolating future consequences from recent observations. The time depth of Fossil Earth helps us to reduce this uncertainty and to refine predictive scenarios.

6In this contribution, I propose to cruise some of Fossil Earth’s worlds. Sepulchre (2015, this volume) conducted a similar exploration following a paleoclimatological approach. Doing this, he showed the connections between physical earth, climate, and biodiversity. This is important, because now at the beginning of the 21th Century, we are not merely facing a climate change. Rather, we are facing a multifactorial and global environmental crisis. There is a strong level of international commitment on the particular issue of climate change because it has been showcased by climatologists with energy and soundness, but in this crisis climate is only one component among others.

  • 3 Of course, the on-going climate warming is itself of anthropic origin (IPCC, 2014). And even if it (...)

7Within our observable part of the universe, Earth’s biosphere is unique. As with climate, the biosphere constantly evolved through time, responding to physicochemical factors (abiotic parameters, including climate) as well as to its own action (biotic interactions). Biotic interactions are a key aspect of the on-going crisis. Exhaustion of marine resources by overfishing, land salinization by intensive irrigation, damage and pathologies brought by invasive species, decline in abundance of pollinating insects, and deforestation are examples of the major issues directly linked to biotic interactions – primarily interactions between our species and the rest of the planet. The impacts of these actions can be reinforced by climate warming, but they can also intensify the speed and harmful consequences of the latter3.

8Our planet has already gone through many large-scale environmental crises. The five most dreadful, called mass extinctions, resulted in the collapse of whole ecosystems at global scale and the extinction of most existing species within a relatively short time, generally less than two million years. Barnosky et al. (2011) asked the following question: are we now facing the sixth mass extinction in the history of our planet? They estimated that, given current trends, 75 % of living species should disappear in less than 11,330 years in the best-case scenario, and in less than 250 years at worst! Ceballos et al. (2015) suggested that the current extinction rates are eight to 100 times faster than the usual “background rates.” So, these studies clearly lend support to the concept of “sixth mass extinction.”

9This paleontological exploration of the Fossil Earth first examines some of the previous mass extinctions. This should help in grasping their general traits which could also characterize the on-going biodiversity breakdown. The second half of my contribution deals with the tiny portion of Fossil Earth (0.15%) inhabited by humankind. Humankind has never before experienced a mass extinction, but the available fossil records and methodologies make it possible to observe our ancestors’ responses to past environmental changes. The work currently being carried out in the Lower Omo Valley in Ethiopia illustrates this. In addition, even if major aspects of our evolution are still poorly understood, it is possible to learn some lessons from the travel of humankind through the Fossil Earth.

Serial apocalypses

  • 4 Biotic crisis is another frequent name for mass extinction events. The big five were: the final Ord (...)

10The five greatest biotic crises4 have all resulted in the loss of at least 75% of existing species. These events lasted from more than 20 million years to less than 100,000 years, if not (in one case) in less than a year! They were all triggered by more or less well understood combinations of factors, which all seem to differ from each other in details. Following are depictions of three of these events, starting with the biotic crisis that occurred during the Late Devonian, around 372 Ma (the Mega-annum, or Ma, is the international system unit used for dates in million years).

The Frasnian-Famennian crisis

11Barring the quite unlikely release of a “Spielbergian” Devonian Park, the Devonian (419 Ma – 359 Ma) will remain a period largely unknown to the general public. Yet, it was a turning point in life history. During the Devonian, terrestrial ecosystems gained momentum, notably with the establishment of the earliest forests (e.g., Meyer-Berthaud & Decombeix, 2012)5 and the appearance of the two most important groups of terrestrial animals: the insects (Glenner et al., 2006) and the tetrapods (Clément & Letenneur, 2009). Today, the former group has the greatest land diversity and the heaviest land biomass, and plays a vital role in ecosystem life (notably by pollinating flowering plants). The earliest known insects were Devonian terrestrial forms.

12The latter group included the ancestors of all extant land-based vertebrates, including Homo sapiens. Unlike fishes, the first Devonian tetrapods, all aquatic, displayed two pairs of limbs with digits, which later allowed their descendants to invade land6. The Devonian tetrapods were relatively discreet, and the dominant vertebrates and the true rulers of the Devonian seas were the placoderms, a major group of armored fish and also the earliest jawed vertebrates, including the spectacular Dunkleosteus7.

13This distant Devonian world lost ca. 75% of its species before 359 Ma. The tropical marine environments were by far the most severely impacted. The rich coral and sponge reefs were destroyed and the placoderms met their end. On the continents, many plants disappeared, while freshwater tetrapods became so few that they literally vanished from the fossil record until much later.

14The duration and complexity of this event distinguish it from other mass extinctions. It happened in several waves spread over more than 20 million years. The main peak, centered on the transition between the Frasnian and Famennian ages (372 Ma), lasted for about 1.5 million years. The proposed causes are diverse, including a succession of meteorite or comet collisions with Earth. However, the most likely direct causes were a climate cooling, perhaps involving a glaciation, and a dramatic decline of marine water oxygen – or anoxia (McGhee in Briggs & Crowther, 2001: 223).

  • 8 Carbon sequestration is the trapping of carbon in non-atmospheric forms.
  • 9 This process is the reverse of the current global warming linked to increasing atmospheric greenhou (...)

15These immediate causes, however, could have had a single source: the Devonian forests (Algeo & Scheckler, 1998). The rapid expansion of the earliest forests and the root penetration of their trees led to the formation of thick soils. These soils contributed to substantially modifying the patterns of land erosion, triggering a massive input of nutriments into continental and marine waters. Resulting massive algal blooms probably caused the observed global marine anoxia. In addition, Devonian forests pumped out a lot of the atmospheric CO2, sequestering it in soils8. This decrease of atmospheric CO2 may have led to global cooling9. After the Devonian, the establishment of an atmospheric cycle integrating the plant sequestration of CO2 restored the balance.

The Great Dying

16Contemporaneous with the onset of the supercontinent named Pangea10, the end of the Paleozoic era (541 Ma – 252 Ma) was defined by the greatest mass extinction of all times: the final Permian event, when about 96% of species went extinct. Groups that had populated the seas for almost 300 million years vanished. This is the case of the trilobites11, already weakened by the Late Devonian crisis, and of various large, sessile organisms12 of the sea floors. On land, the main herbivorous and carnivorous species of reptiles13 and amphibians were killed, as well as the main forest tree species. And this was the only time that insects were strongly impacted by a biotic crisis (Wignall in Briggs & Crowther, 2001: 226).

  • 14 Traps are successive lava flows that formed stairstep landscapes (from the Scandinavian root “trapp (...)

17Once again, the direct causes of this event were probably multiple (Lane, 2007), including a dramatic decrease in atmospheric and marine oxygen content, an increase of greenhouse gases (CO2 and probably methane), and local abundances of sulfur gases in seas. Global temperatures rose by 6 °C, terrestrial and marine biotas suffocated or were even poisoned by sulfurs and acid rains, while the acidification of the seas compromised the development of hard tissues by marine organisms. The primary culprit of these cataclysmic conditions is most likely immense volcanic activity, knocking out environments relatively weakened by rather low oxygen concentrations and a relatively dry climate. For about one million years, a series of eruptions released 3,000,000 km3 of basalts forming the Siberian “traps”14 and a huge quantity of greenhouse gases and atmospheric acids, also possibly altering the ozone layer.

18A particularly striking aspect of the terminal Permian event is the rhythm of ecosystem recovery. The redevelopment of stable and complex sea floor ecosystems took about nine million years (Chen & Benton, 2012). In the aftermath of the killing, the sea floors were mainly covered by microbial crusting. Later, opportunistic organisms (notably bivalve seashells) gained ground, forming extremely monotonous faunal assemblages attesting to low biodiversity levels15. This situation lasted for several million years.

  • 16 Notably crocodilians, flying reptiles (pterosaurs), dinosaurs, and birds.

19This extinction largely shaped the post-Permian Fossil Earth. Indeed, the surviving organisms were those able to live despite low oxygenation. Sea floors became dominated by burrowing or wandering mollusks (bivalves and gastropods) and decapod crustaceans (crabs, shrimps, etc.), a structuring which still prevails today. The decimation of Paleozoic land tetrapods favored the emergence of new major groups that became the rulers of the succeeding eras: marine reptiles, archosaurs16, and mammals. The greatest mass extinction of all times hence triggered the evolution of more complex, further diversified, and more efficient organisms.

The K/T crisis

  • 17 K/T stands for Kreide/Tertiär in German, i.e. Cretaceous/Cenozoic or Cretaceous/Tertiary.

20The K/T17 boundary separates the Cretaceous (145 Ma – 66 Ma) from the Cenozoic era (66 Ma – present). During the Mesozoic era (252 Ma – 66 Ma), the seas were dominated by ammonites and marine reptiles, while archosaurs ruled on land and in the air.

21The ammonites18 were part of the same biological group as octopuses and squids (the cephalopods), but they differed from these in having a chambered spiral shell allowing precise control of vertical motion, like the use of ballast in submarines. These animals, living in the depths as well as near the shores, occupied an intermediate, particularly rich level of the oceanic food chain. Various marine reptiles19 crowned this food chain, such as ichthyosaurs20, which were fully adapted to aquatic life in the same way living cetaceans are. At the bottom were planktonic and coralline organisms, extremely abundant in Mesozoic epicontinental seas, as indicated by the thick calcareous and chalky rocks they formed.

22Archosaurs had quickly diversified during the early Mesozoic. Among them, the famous dinosaurs21, including the largest land animals ever, proliferated during the Jurassic (201 Ma – 145 Ma) and the Cretaceous. Despite popular imagery, we now know that these animals were not archaic and stupid, but in fact formed complex ecological communities and could display elaborated behavior such as gregariousness and parental care. Another major success of the Mesozoic archosaurs was the conquest of the air. This was performed by the pterosaurs (or flying reptiles)22 and by a group of dinosaurs, the Maniraptora, from which the birds emerged23.

23This Mesozoic World, densely populated and well-structured, was overturned by the most famous of all mass extinction events (rate of species extinction: 76%). Among other groups, the dinosaurs died out at this time. Again, this event was probably caused by a combination of catastrophes. Around 66 Ma, a new sequence of volcanic eruptions build the Deccan traps in India, piling basalts over more than 2 km in some places. Although less massive than the final Permian traps in Siberia, the Deccan traps formed in a shorter time, and some of these eruptions may have released huge quantities of sulfur gases into the air within years or decades (Archibald et al., 2010; Courtillot & Fluteau, 2010; Keller et al., 2010). Ecosystems were probably already quite weakened by this volcanic pollution when they received the coup de grace: a 10-km-diameter asteroid fell near the northern tip of the Yucatán Peninsula in Mexico, creating the Chicxulub crater with a diameter of ca. 180 km.

24This impact generated a global shock wave with giant fires, and above all sent into the atmosphere hundreds of millions of tons of dust and no less than one hundred billion tons of sulfur gases, occulting sunlight for a long duration (Schulte et al., 2010). The resulting “winter” was probably uninterrupted for several years. Plankton, corals and land vegetal cover were largely destroyed. Land masses were for a time covered by fungi, then by ferns. All the dominant groups described above were exterminated, except the avian branch of the dinosaurs.

25Again, this mass extinction event helped outsiders to replace the former rulers, mammals being a very good example. Mammals appeared during the Mesozoic at ca. 225 Ma, i.e. only a few million years after the dinosaurs. Although important phases of their phylogenetic and ecological diversification took place during this era, they remained discreet and largely overshadowed by archosaurs in the ecosystems. After the K/T crisis, however, the surviving mammals went through an impressive diversification and came to replace the missing archosaurs and marine reptiles. This trend culminates today with the grip of humankind on the entire planet.

Exegesis of biosphere serial apocalypses

26This quick survey of three great biotic crises indicates that if some crises had external primary causes (e.g., volcanism and meteorites), others had internal causes: in other words life destroyed life. This is likely the case with the rapid expansion of late Devonian forest ecosystems, the primary source of a crisis that terminated 75 % of living species. I would like to mention another, much more ancient example. From ca. 3,800 Ma onward, unicellular photosynthetic organisms started to release massive quantities of oxygen (O2) in the Precambrian environments. Today vital for most living beings, oxygen was then toxic to most organisms which had an anaerobic metabolism. At first all this oxygen was captured in the process of oxidizing oceanic iron. By 2,400 Ma, this process was largely complete, and the oxygen could now be released as free oxygen into the seas and the primitive atmosphere. This triggered an ecological crisis during which opportunist aerobic life forms (using O2 in their metabolism) put an end to 1.5 billion years of dominance by anaerobic organisms. Therefore, life has been able to deeply and lastingly modify the physicochemical and biotic conditions prevailing on Earth’s surface.

27Whatever tremendous biotic crises occurred, life always recovered and even more. After each sinking of a Fossil Earth world, the emerging world was more complex, lusher, and more sparkling. Yet, this was far from being an easy process: for every single major environmental crisis, the planet remained tarnished for a long time, and the emergence of bright new worlds always took millions of years (possibly about 10 after the Permian, and at least five after the K/T crisis).

28These recovery phases did not benefit the pre-crisis dominant organisms, however complex and efficient they may have been. On the contrary, these organisms were generally the most severely affected. The extinction of non-avian dinosaurs is a typical case: despite their dominant position, they appeared more sensitive to abrupt changes than relatively more discreet groups, such as mammals.

29It is tempting to explain the favorable fate of mammals and birds at the K/T boundary because they seemed more gifted than dinosaurs: they were small-sized, warm-blooded, smart, and occupied peculiar ecological niches (burrowing, arboreal, or flying). However, this explanation does not hold up to scrutiny. Indeed, many species of mammals and birds perished during the K/T crisis, while other groups lacking these alleged advantages did survive, such as crocodilians. In addition, we now know that small size, efficient thermoregulation as well as complex behaviors and ecologies also characterized various dinosaurs and pterosaurs.

30Instead, I would like to suggest that the very dominance of dinosaurs, ammonites and marine reptiles could have been their main weakness as well. Thanks to their morphological and physiological innovations evolved during the early part of the Mesozoic, these animals gained privileged access to the most abundant and the highest quality of primary resources. Because of this preemption, Mesozoic ecosystem structures were influenced by the ecological characteristics of these dominating groups. A high level of competition within these groups was supported by high levels of biomass consumption, favoring lush evolution and long-term ecological dominance. But it is possible that such important energy needs made them eminently sensitive to extreme scarcity situation. With the K/T apocalypse, the collapse of most primary producers led to the downfall of the primary and secondary consumers that had been best integrated within the Jurassic-Cretaceous ecosystems. By contrast, the most marginal fringes of these ecosystems, more often exposed to resource scarcity or to low quality resources, were probably more efficient in responding to a catastrophic change. This was notably the case of fungi, ferns, some mammals, some birds, some crocodilians, etc.

31Finally, one can note that during the ~135-million-year rule of non-avian dinosaurs, no other group superseded them. If Mesozoic conditions had remained more or less stable, it is quite likely that this ecological balance would have lasted another 66 million years. Consequently, Earth would be currently dominated by further evolved, non-flying dinosaurs, and mammals would have never experienced the evolutionary radiation that led to horses, elephants, cetaceans, and chimpanzees. In other words, we are the children of the K/T event…

Humankind24 facing environmental changes in the Fossil Earth

  • 24 Comparative anatomy and molecular biology have formally demonstrated that the closest living specie (...)
  • 25 kilo-annum, for dates in thousands of years.

32All fossil witnesses of humankind older than 1.8 Ma have been discovered in Africa. This is true in particular for the most archaic such remains, dated to between 7 Ma and 4.4 Ma and characterized by frequent bipedalism and relatively small-sized canines (Senut et al., 2001; Brunet et al., 2002; Haile-Selassie et al., 2004; White et al., 2009). Later on, most of the main events in our evolution also took place in Africa, including the emergence of our own species at ca. 200 ka25 (Day, 1969; White et al., 2003; and for a summary, Boisserie, 2011). “Fossil Africa” is therefore a key part of the Fossil Earth for observing the long-term impact of environmental changes on humankind.

  • 26 Formed by the rise of a ca. 2,000-km-wide bubble of magma under eastern Africa. This created a domi (...)

33For a long time, scientists have looked for correlations between environmental events and the main phases of human history. Two hypotheses gave assigned central role to abiotic factors of regional or global scale. The first one, nicknamed “East Side Story,” aimed at explaining the origin of bipedalism (e.g., Coppens, 1994). In this scenario, the formation of the Arabo-African rift system26 drove the drying out of eastern Africa. The resulting opening up of landscapes selected eastern African hominoid primates more able to move efficiently outside forests, i.e. those that developed bipedalism and that h the earliest representatives of humankind. Some aspects of this hypothesis have since been validated, notably the climatic impact of the rift formation (Sepulchre et al., 2006). However, the discovery of fossil humans in central Africa, older than the eastern African ones (Brunet et al., 2002), as well as the reconstruction of wooded habitats for ancient human species in eastern Africa (WoldeGabriel et al., 2001; White et al., 2009), disagree with a simple causal relation between the tectonics-climate coupling and bipedalism.

34The second hypothesis correlates the emergence of genus Homo and the ability to make stone tools with a global climatic cooling that led to further aridity and thus an even greater opening up of African landscapes (Coppens, 1975). At a more general level, this hypothesis links the evolution of African fauna (including humans) to global climatic changes (e.g., deMenocal, 1995; Behrensmeyer, 2006; deMenocal, 2011). In its various versions, it puts forward single events (Vrba, 1995), climate variability (Potts, 1996), or cyclical phenomena (Trauth et al., 2005) as the main drivers.

35All these hypotheses face a difficulty in establishing strong enough correlations between the record of environmental evolution and that of biotic evolution (e.g., White, 1995). This requires particularly precise records, but also the examination of multiple factors, inasmuch as a good correlation does not necessarily warrant a causal relationship. In addition, human remains are particularly scarce, even in the giant fossil accumulator constituted by the eastern African rift. If, after decades of research, this record still suffers from many gaps, we nonetheless do have good assets for improving the situation. I propose to describe one example of these assets: the Shungura Formation in the Lower Omo Valley, southwestern Ethiopia.

A natural laboratory for testing the human-environment interactions in the past

36At ca. 2.8 Ma, our evolution experienced a major twist: the emergence within humankind of two major parallel branches. On the one hand, robust australopithecines were displaying particularly strong jaws and molars compared to other humans27; on the other hand, the forerunners of genus Homo, to which we belong, had slender jaws and a supposedly more developed brain. Roughly at the same time, the cycle of northern hemisphere glaciations was beginning and the global climate became more unstable and cooler, possibly with more marked seasons, all this resulting in more open landscapes. Many scientists see a causal relationship between this climatic change and the concurrent evolution of robust australopithecines and forerunners of Homo.

37The Shungura Formation provides an excellent record of these events. Most other contemporaneous deposits in Africa are limited, characterized by a reduced spatial extension and/or gaps in the record. To the contrary, the deposits of the Shungura Formation crop out over an area of on ca. 160 km² and constitute a particularly well-dated, thick and continuous sequence dated to between 3.6 Ma and 1 Ma (Heinzelin, 1983; Feibel et al., 1989). This sequence is rich in remains of the past: about 49,000 fossils of vertebrates were discovered during the 1960s-1970s, including many ancient human remains as well as stone tool series which are among the oldest known tools (ca. 2.3 Ma). Given its extension, its temporal depth, its continuity, its richness and the nature of its fossil record (Coppens et al., 1976; Boisserie et al., 2008), the Shungura Formation can be seen as a real “natural laboratory” where evolutionary dynamics, their causes and their impacts can be observed within the “Fossil Omo Valley.”

38Whereas existing museum collections are often used to establish correlations between global climatic changes and faunal changes (e.g., Bobe & Eck, 2001), new fieldwork we have performed at Shungura since 2006 (Boisserie et al., 2008; Boisserie et al., 2010) has been much more narrowly focused, specifically targeting the impact of local scale factors on the Omo Valley ecosystem. This includes the human populations as well as the biotic and behavioral responses of these populations to the evolution of local factors.

39For this, our team first had to expand the available data on human evolution in the Omo. Its work has resulted thus far in the discovery of 35 human fossils, including isolated teeth, jaws, and limb bones. The study of these fossils should help clarify the diversity of our ancestors, in particular the identity of the forerunners of genus Homo at ca. 2.5 Ma. At the same time, new research on the stone tool industry produced by these populations helps to understand their behaviors and cultures (Delagnes et al., 2011; Maurin et al., 2014).

40Another goal of our work is to characterize as accurately as possible the local ecosystem and its evolution from 3.6 Ma to 1 Ma. This involves first the examination of sedimentary deposit features for reconstructing the environmental conditions that resulted in their formation (e.g., meandering rivers, deltas, or deep lakes). This also includes larger scale dynamics that, through time, shifted these environmental conditions across the landscapes.

41The past biodiversity of the Omo Valley is another critical corpus of data. The analyses of this biodiversity, reinserted into its geographic, temporal and environmental context, focus primarily on key vertebrates, such as antelopes, pigs, hippos, fish, monkeys, and of course humans. Not only do our analyses contribute to documenting faunal changes at ecosystem scale, but they also target the evolutionary history of each individual group, exploring different modes and factors of evolution.

42Finally, the reconstruction of the Omo Valley past environments involves reconstructing the vegetal cover. This can done partly by using direct evidence, such as fossil wood, pollens, silica particles formed in vegetal tissues (phytoliths), and the biogeochemical compounds preserved in fossil soils (for an example combining these methods, see Barboni, 2014). Indirect evidence can be also used: the diet of past herbivorous animals can be estimated by quantifying the biogeochemical compounds found in their tooth enamel, as well as the microscopic wear marks on teeth which vary from one type of food to another. Finally, past seasonal cycles can be observed thanks to the markers of water evaporation, also preserved in the dental enamel of animals that drank or lived in this water.

Observed human responses to past environmental changes

43The Omo Valley presents us with a record of several disruptions in its ecological history. The first one is witnessed by a dietary change in several species around 2.8 Ma (Bibi et al., 2013), likely indicating a change in vegetal cover. This event occurred just before the appearance of robust australopithecines in the valley and, possibly, of the forerunners of genus Homo.

44The second disruption is a major faunal turnover28 at ca. 2.4 Ma, preceding by 100,000 years the sudden appearance of stone tools in the valley. These tools, made from quartz pebbles that were carefully selected, immediately became abundant in the landscape (Delagnes et al., 2011). At the same time, robust australopithecines further specialized, their cheek teeth becoming even larger and their incisors and canines even more reduced29.

45The third disruption seems to correspond to a biogeographic isolation of the valley from 2 Ma onward, suggested by a peculiar endemic fauna (Boisserie, 2013). At the onset of this isolation, the Lake Turkana waters drowned the Omo Valley, then receded. Stone tool making was mostly or totally stopped after 2 Ma, although robust australopithecines and Homo inhabited the valley continuously until 1.4 Ma. At this date, robust australopithecines vanished, Homo remaining alone in the valley.

46This sequence of events is an opportunity for testing the mechanisms and factors controlling ecological changes, biological evolution, as well as the acquisition and loss of technological activities in this valley. In fact, the Shungura evolutionary record illustrates the various categories of human responses to the environmental challenges that occurred throughout its deep history.

47A first category of human response is the adaption to environmental changes through innovations. Innovations have been morphological, physiological and ecological: e.g., when habitats opened up and dried out, the strong teeth and jaws of robust australopithecines made it possible to feed on hard and/or abrasive foods at least occasionally (Ungar, 2011). The slender body acquired by some representatives of Homo by 1.8 Ma (e.g., Lordkipanidze et al., 2007) is another example of adaptive innovation: in the context of a retreating or vanished forest cover, it probably made it easier to travel long distances under the sun and the eyes of predators. To the contrary, the stocky body of the Neanderthals (known from ca. 400 ka to ca. 40 ka from western Europe to central Asia) was well-adapted to cold climate (Weaver, 2009). Innovations were also behavioral (e.g., Foley & Gamble, 2009), including notably the early development of stone industries (Semaw et al., 2003; Harmand et al., 2015) and the successive stages of their diversity and complexity.

48Other human responses seem more like reversals than innovations. Morphologically speaking, the most striking case could be that of Homo floresiensis30. This species, discovered in 2003 on Flores Island in Indonesia and dated to ca. 14 ka for the youngest remains, came as a surprise by virtue of the small size of its braincase and its body, size-wise more similar to human species known in Africa ca. 2 Ma than to recent ones (Brown et al., 2004). What happened to the humans who settled on Flores at ca. 1 Ma (Morwood et al., 1998)? In an insular context, typified by geographic isolation and limited resources, many species have experienced a dramatic decrease in their body size (Sondaar, 1977), and in some cases in brain size (e.g., Weston & Lister, 2009). The Flores humans possibly went through such insular evolution, leading to a pygmy species that, on the brink of our modern world, had an encephalization quotient31 not much different from that of an australopithecine.

49More clear-cut “reversal” cases are known from the techno-cultural record. The case of the Omo quartz industry, i.e. the loss or abandonment on know-how correlated with an environmental change, is not an isolated one. Between ca. 40 ka and 12 ka, the material culture that developed during the last glaciation in Western Europe, called the “Upper Paleolithic,” produced remarkable art pieces, globally renowned thanks to the cave paintings of the Lascaux Cave and the Chauvet Cave32. This culture blossomed within populations that were technically and culturally adapted to the then-cold and dry climate of this region, with an economy principally based on the hunting of steppe mammals. With the quick deglaciation and withdrawal of these animals at ca. 12 ka, the western Europeans were thrown into a new world, the Holocene33. Undoubtedly puzzled by these new conditions and probably pushed back to the brink of survival, these people drastically reduced their artistic practices, now mostly limited to decorated pebbles34.

50Finally, a last type of human response is pure and simple extinction. Humankind has been composed of several contemporaneous species probably for most of its history. Today, there is only one species, which means that other lineages went extinct. The robust australopithecines, living side by side with Homo for more than 1.5 million years, vanished definitively between 1.4 Ma and 1 Ma. Until very recently, three very different groups of humans existed at the same time: Neanderthals, Homo floresiensis, and our species. Neanderthals went extinct by 40 ka, the Flores pygmies by 14 ka. In each case, plausible causes were either abiotic changes (such as the intensification of the last glaciation impacting Neanderthal habitats) or biotic changes (notably, the expansion of Homo sapiens to Western Europe and Flores). As for biotic crises, a combination of these factors is quite likely, and may well explain the extinction of the robust australopithecines in Africa. In this latter case, it is also possible that their morphological specializations had led them to an evolutionary dead end.

Lessons from extinct worlds

51This exploration of the Fossil Earth may tend to reassure incurable optimists about on-going changes. After all, life, toughened by the previous mass extinctions, will most probably go on beyond this crisis too. The massive anthropization of our planet, one might argue, is just another step in its transformation by life: bacteria, then land vegetation, have previously dramatically altered the original state of Earth, so why not us? This is a philosophically interesting viewpoint. However, at the risk of disappointing professional lobbyists for skepticism regarding climate change and citizens irritated by authoritarian ecological dogmatism, it does not have much immediate practical use for us.

52First, the claim of “anthropization as just another step” might well turn against us in a not-so-distant future, on the occasion of Earth’s next evolutionary step made, for example, by artificial intelligences35. Second, if we consider only the reduction of biodiversity, which gets much less coverage in the mass media than global warming, it not only entails great suffering for wild species, but also for humankind and its commensals. This not only implies the erosion of supposedly renewable resources vital to many people (in particular marine and forest resources), but it also involves the dysfunction of major services provided by natural ecosystems (Cardinale et al., 2012; Johnson et al., 2013): carbon sequestration, soil fertilization, pollination, and limitation of pathogens and pests are threatened processes. In addition, one can note that wildlife has always been a great source of inspiration and innovation for human cultural and technological advances. Some examples are Upper Paleolithic art, mostly portraying non-human species36, medical research, and various religious myths. are examples among others. So a decline in biodiversity also means the disappearance of some of our cultural foundations and of sources for future innovations.

  • 37 For learning about potential threats to the conservation of the Omo Valley sites, see: http://whc.u (...)

53Observations of the past in general and paleontology in particular do not provide ready-made solutions for improving the situation. Yet, on the basis of past natural experiments, paleontologists can provide a more robust scientific basis for anticipating faunal and vegetal responses to environmental changes than extremely short-term observations. Investigation of past life therefore plays an increasing role in the process of decision making for preserving extant biodiversity (Dietl & Flessa, 2011; Blois et al., 2013). There is room for significant improvement in this area, notably thanks to renewed multidisciplinary studies of exceptional sites, in particular in Africa, such as the Lower Omo Valley. In this regard, heritage management is a key aspect of such future researches37.

54Scrutiny of the Fossil Earth finally offers a general framework for thinking, without which what is at stake hardly makes sense for too many of us38. I would like to conclude by sketching this framework with some general remarks based on the observation of deep past life.

55Paleontology reveals the considerable impacts of environmental change on life over the course of its history. We live in a constantly changing world, a fact which is not obvious on the scale of a human life.

56Compared to the background rate of past changes, the on-going changes are close in their speed and intensity to the most sudden and catastrophic events of all of Earth history (Barnosky et al., 2011; Ceballos et al., 2015; Sepulchre, 2015, this volume). In this and the previous regard, paleontology can play an educational role for understanding and accepting that global environments are currently evolving in a truly impressive way.

57From its earliest development, life has had a large scale effect on the biosphere and on the physicochemical parameters of Earth’s surface. The ecological balance prevailing on our planet is indeed so fragile that, in past times, it was totally overturned by microscopic unicellular organisms and by plants. Our species, densely distributed and endowed with rich cognitive and technological capabilities, is thus able to have a deep, swift and long-lasting effect on this balance. In other words, those who do not believe that humankind or other living organisms could be the cause of a large-scale environmental crisis are plainly wrong.

  • 39 By the way, this reasoning can be extended to oppositions between different human populations, give (...)

58The idea of an opposition between humankind and nature, deeply anchored in our mentality, is overall detrimental in terms of environmental crisis management, whether it serves to justify 1) unrestrained exploitation of natural resources, or the opposite 2) demonization of all or part of humankind (Blanc, 2015). Paleontological data, together with biological data, demonstrate that this nature/humankind opposition is devoid of any historical or ecological foundations. Our ancestors’ fossils scattered over the Fossil Earth (and notably “Fossil Africa”) deeply root extant humankind within the biosphere history, our species having been born from the same process as all other living beings. In addition, Homo sapiens is not the only species to have developed advanced cognitive abilities, as indicated by the astonishing technical and cultural traits of other species such as apes and crows (e.g., Emery & Clayton, 2004; Caron et al., 2011; Roffman et al., 2012). It is therefore imperative to consider the whole biosphere, humankind included, as an integrated system39.

  • 40 A complete extinction of Homo sapiens would be unlikely to occur soon, given the size of our global (...)

59As with all other living beings, the fossil record indicates that past human populations went through declines and/or extinction. The progress of humankind is not irresistible, and the emergence of morphological and behavioral innovations has been related to environmental factors, the very complex cultural behavior of Homo sapiens being no exception to the rule (d'Errico & Stringer, 2011). So it must be clear to everyone that the on-going environmental changes could really cause decline, collapse (Diamond, 2005), and even, locally40, extinction of human populations.

  • 41 In my opinion, the motto “Save the Earth” is misleading: our living planet has already gone through (...)

60Recovery phases of global ecosystems following previous major biotic crises required millions of year. Accordingly, given the temporal scale of a human life (at an average of 70 years), of a human society (some hundred, or even thousand years), or even of our species (200,000 years), to wait for a natural recovery of these ecosystems, or waiting for their adaptation to human activities, is not an option for us41.

61The greatest biotic crises were triggered by a combination of cumulating factors, sometimes amplifying each other. This is what is currently observed, with the concomitance of global warming, of biodiversity decline, and of the explosive increase of global demography. This combination must be taken into account in the actions that should be decided. Following an analogy with the K/T crisis, discussing on climatic issues alone would be like fearing volcanoes while believing meteorites are fairytales. Given the intertwining of the problems, solutions need to be integrative.

62Finally, one can note that, for a given species, its fate during a biotic crisis depends on the nature of its adaptability. Resilience, i.e. the ability to adapt to environmental pressures so as to preserve abilities and/or adaptive traits insuring a favorable status, is an important aspect of adaptability. Hence, non-avian dinosaurs were remarkably resilient, as they were able to constantly renew their initial adaptive recipe for a successful exploitation of Mesozoic ecosystems for almost 150 million years. By comparison, the adaptability of mammals was clearly less efficient in the same evolutionary context.

63However, another adaptive quality is to be able to use its initial abilities in a totally new framework. The sudden collapse of the main Mesozoic primary producers led to the collapse of the best integrated components of the trophic networks. To the contrary, some mammals survived through that event and then thrived because they had the potential to free themselves from the framework of Mesozoic ecosystems and to survive in drastically different conditions. Similarly, the only surviving dinosaurs, i.e. birds, were those that followed a very peculiar evolutionary pathway among the group.

64It is very tempting to draw a parallel between this scenario and the current situation of humankind. Our advanced societies are based on a complex exploitation system of natural resources, which has been developed in the context of the relatively stable environmental conditions that have prevailed for about the last twelve thousand years. These societies went through spectacular changes notably thanks to emulation and competition, and to an exponential growth in their energy consumption. They are therefore in a position recalling that of non-avian dinosaurs, and their strength could potentially be their greatest weakness in case of catastrophe.

  • 42 For other examples, see Diamond (2007, 2009, 2010).

65Mammals in general and humankind in particular display remarkable abilities to adapt, and at least some of us have an acute awareness of the on-going environmental crisis. However, modern societies have grown considering the world as if it was endlessly vast with almost unlimited resources, and transmitted this assumption to the “social genome” of our extant societies. Collectively, humans are bound by relatively rigid cultural and societal constraints. The extinction of the Norse colony settled in Greenland from the 10th century to the 15th century is likely an unfortunate example of combination between social rigidity and inexorable environmental change (Barlow et al., 1997)42.

66Our societies are therefore facing the advent of a new world. Should we keep up with most of our past successful recipes, or should we opt for something radically new? For living species, the former solution most generally has ended in extinction when changes were quick and massive. The latter solution requires the ability to divert an adaptive trait from its initial function toward a fundamentally different function – what paleontologists Gould & Vrba (1982) have called “exaptation.” In short, we, the offspring of the K/T crisis, must remember that all will depend on our ability to use the upheaval of our planet as an opportunity to further evolve.

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Notes

1 More pragmatically, Pratchett & Baxter’s multiverse (2012), by providing millions of human-empty worlds alongside our own iteration, would resolve a frightening dilemma of our actual world: according to the ecological footprints computed for the wealthiest countries, the lasting adoption of their ways of life by all ~7.3 billion humans would be possible only by consuming the natural resources of at least four more additional earths (cf. http://www.footprintnetwork.org/en/index.php/GFN/).

2 For a geological time scale, see: http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.jpg

3 Of course, the on-going climate warming is itself of anthropic origin (IPCC, 2014). And even if it was not, the observed climatic trends would threaten our future all the same, and it would be certainly not less urgent to act against them!

4 Biotic crisis is another frequent name for mass extinction events. The big five were: the final Ordovician event, ending at ca. 443 Ma; the Frasnian-Famennian event (Late Devonian), ca. 359 Ma; the Permo-Triassic event, ca. 252 Ma; the final Triassic event, ca. 201 Ma; the K/T event, at 66 Ma. Illustration: https://commons.wikimedia.org/wiki/File:Extinction_Intensity.svg

5 For a reconstruction of a Devonian forest: https://www.youtube.com/watch?v=KJdCytwAigU

6 Illustration: http://evolution.berkeley.edu/evolibrary/images/evograms/tetrapod_evo.jpg

7 Reconstruction (body length ca. 10 m): https://upload.wikimedia.org/wikipedia/commons/4/4c/Dunkleosteus_interm1DB.jpg

8 Carbon sequestration is the trapping of carbon in non-atmospheric forms.

9 This process is the reverse of the current global warming linked to increasing atmospheric greenhouse gases.

10 Illustration: http://cpgeosystems.com/240moll.jpg

11 Illustrations: http://static.palaeontologyonline.com/Figure13.jpg and http://www.trilobites.info/triloclass2009.png

12 Reconstruction: http://science.nationalgeographic.com/staticfiles/NGS/Shared/StaticFiles/Science/Images/Content/permian-diorama-umich-img-4260-sw.jpg

13 Reconstruction of some species: http://geologicalman.blogspot.es/cache/media/files/00/736/872/2015/05/1-permian-animals-artwork-mauricio-anton.jpg

14 Traps are successive lava flows that formed stairstep landscapes (from the Scandinavian root “trapp” for stairs). In some cases, the trap volcanic rocks can pile up until forming several-kilometers-thick sequences.

15 Illustration: http://www.nature.com/ngeo/journal/v5/n6/fig_tab/ngeo1475_F3.html

16 Notably crocodilians, flying reptiles (pterosaurs), dinosaurs, and birds.

17 K/T stands for Kreide/Tertiär in German, i.e. Cretaceous/Cenozoic or Cretaceous/Tertiary.

18 Reconstruction: http://www.nationalgeographic.com/seamonsters/images/gallery/ammonites-lw.jpg

19 Illustration: http://eis.bris.ac.uk/~ts0438/images/Marine%20reptiles.bmp

20 Reconstructions of ichthyosaurs: http://www.dinofan.com/dfAnimals/dfAnimalsSitePics/CladImages/Ichthyopterygia.jpg

21 Illustration: https://commons.wikimedia.org/wiki/File:Various_dinosaurs.png

22 Reconstructions: http://img03.deviantart.net/f23e/i/2015/108/5/2/pterosaurs_by_atrox1-d757xt2.jpg

23 Illustration: https://upload.wikimedia.org/wikipedia/commons/9/9d/Archaeopteryx_lithographica_%28Berlin_specimen%29.jpg

24 Comparative anatomy and molecular biology have formally demonstrated that the closest living species to our species are Pan paniscus and Pan troglodytes, i.e. the bonobos and the chimpanzees, respectively. These species and we share an ancestor more recent than all the other ancestors we share with other living beings. Two evolutionary branches were born from this common ancestor: one is represented today by the genus Pan, the other by Homo sapiens. In this contribution, all extinct representatives of the latter branch are included within humankind.

25 kilo-annum, for dates in thousands of years.

26 Formed by the rise of a ca. 2,000-km-wide bubble of magma under eastern Africa. This created a doming of the earth crust, the formation of the Ethiopian traps, and a collapse of Earth’s crust along three main lines: the Red Sea, the Gulf of Aden, and the eastern African rift running from the Afar triangle (Djibouti and Ethiopia) to the coast of Mozambique.

27 For further information and illustrations: http://humanorigins.si.edu/evidence/human-fossils/species/paranthropus-aethiopicus

28 Many species go extinct while many others appear in the fossil record.

29 For further information and illustrations: http://humanorigins.si.edu/evidence/human-fossils/species/paranthropus-boisei

30 Illustration: https://commons.wikimedia.org/wiki/File:Homo_floresiensis.jpg

31 Ratio between brain mass and whole body mass.

32 www.lascaux.culture.fr and http://archeologie.culture.fr/chauvet/fr/

33 The Holocene started at 11.7 ka and continues today. It is a relative stable, interglacial climatic phase during which Homo sapiens developed an economy based on controlled production, complexly structured civilizations, and large populations.

34 https://commons.wikimedia.org/wiki/File:Galet_peint_MHNT.PRE.2006.0.93.jpg

35 See, e.g., some serious recommendations and caveats on this topic: http://futureoflife.org/AI/open_letter

36 www.lascaux.culture.fr and http://archeologie.culture.fr/chauvet/fr/

37 For learning about potential threats to the conservation of the Omo Valley sites, see: http://whc.unesco.org/document/127576

38 An example: http://www.debate.org/opinions/do-you-believe-in-global-warming

39 By the way, this reasoning can be extended to oppositions between different human populations, given that our own species was born from a single, extremely recent African population (on the scale of the history of life, 200 ka is like 26 minutes to a year).

40 A complete extinction of Homo sapiens would be unlikely to occur soon, given the size of our global population. Yet, in case of global decline, our species would probably become much more vulnerable to adventitious catastrophes (asteroid, nuclear war, pandemic diseases).

41 In my opinion, the motto “Save the Earth” is misleading: our living planet has already gone through the worst and did not need us to survive, thank you very much! A better statement would be “Save yourself, you fool!”

42 For other examples, see Diamond (2007, 2009, 2010).

Auteur

CNRS (CFEE & IPHEP). Head of the Omo Group Research Expedition.

© Centre français des études éthiopiennes, 2016

Conditions d’utilisation : http://www.openedition.org/6540

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