Chapter 19

Reticulate Evolution Through Turbulent Times

Evolution takes the form of changes in lineages through time. These changes are captured by names given to species, traditionally defined morphologically with reference to type specimens.1 A conceptual structure has been developed for identifying features that have been derived from some ancestral form, perhaps shared with sister species, and those that are novel, distinguishing the species. Supporting this approach is the biological species concept, based on whether individuals can interbreed and produce fertile offspring. If they can, they share a gene pool and their populations follow a common evolutionary trajectory. Populations may become isolated if males and females fail to recognise one another as mates or if the offspring produced are not viable, for whatever reason. Genetic divergence, through the accumulation of mutations over time, however neutral in their effects, can be used to infer that populations have been isolated for sufficiently long to be assumed distinct at the species level. Of course, one cannot be sure that they would not interbreed if they came into contact later. A similar caveat applies spatially. Should populations separated geographically be regarded as distinct species, or merely subspecies, morphologically different in some features but without barriers to mating if connected? Ecologically, species can coexist only if they differ sufficiently in niche occupation as defined by resources exploited, physiological tolerance or mechanisms for evading predation. Some of these issues were addressed in Chapters 10 and 14 with respect to Africa’s ungulates.

But looking far back in time, all we have to go by are the morphological features shown by fossilised remains, generally fragmented. Almost invariably, these features change through time. Paleontologists must judge whether the morphological changes are sufficiently meaningful to define new species, or perhaps even new genera, indicating more fundamental changes in lifestyles or ‘grades’. Species come and go, but many of these disappearances are not extinctions, merely the transformation of one ‘chronospecies’ into another over time. There is a need to distinguish genuine extinctions when lineages disappear, leaving no descendants, from persistence in somewhat changed form. Some branches of the evolutionary tree terminate while others ramify. Drawing on these concepts, we seek the primate antecedent that gave rise to both humans and chimpanzees, and earlier also gorillas. The further back in time we go, the more branches coalesce.

Figure 17.1 shows around 20 hominin species eventually culminating in only one, but many of the name changes distinguish chronospecies while others represent geographically separated counterparts. The most remarkable feature is how a connecting lineage has propagated unbroken through time, adapting sufficiently to changing environments to warrant name changes at various stages, to culminate in Homo sapiens. How was this brought about, despite huge environmental fluctuations involving shifts in temperature, precipitation, vegetation cover and forms of coexisting animals? Actually, it happened because of these fluctuations, and particularly those distinctive of Africa, as this chapter will explain.

Eight major adaptive transitions can be recognised in the lineage linking some forest-dwelling ape to modern humans:

1.The emergence of the distinct genus Ardipithecus with features facilitating bipedal locomotion, around 5.7 Ma.

2.The transformation of Ardipithecus into Australopithecus with greater bipedal competence, by 3.5 Ma.

3.The separation of Homo, with more generalised dentition supported by a stone tool kit, from Paranthropus with dental specialisations for chewing tough foods, around 2.8 Ma.

4.The appearance of H. erectus/ergaster with fully competent walking and substantially bigger brain, accompanied by shaped stone implements, around 1.7 Ma.

5.The surge in brain volume towards that of modern humans via H. heidelbergensis, beginning around 0.8 Ma.

6.The establishment of a finely crafted tool kit, plus other cultural artefacts, by big-brained ‘modern’ H. sapiens, between 600 and 200 ka.

7.The development of projectile points launched from bows and daubed with poison in finely elaborated tool kits around 70 ka, predisposing the expansion of humans beyond Africa.

8.The spread of pastoralism and cultivation through Africa after 5 ka.

It is apparent that these adaptive shifts tended to be coupled with times of major environmental transitions: (1) establishment of seasonally dry savannas between 12 and 5 Ma, (2) expansion of C4 grasslands during this period, (3) establishment of recurrent glacial advances bringing greater aridity around 2.6 Ma, (4) tectonic disruptions accentuating local aridity ~1.8 Ma, (5) widening climatic oscillations after 0.8 Ma, (6) extremes of aridity during the last-but-one glacial maximum 140 ka, and (7) amelioration of local aridity during the Holocene after 20 ka (Figure 19.1). It is intriguing that there are blanks in the fossil record just around the times when some of these transitions took place. Unconformities in sedimentary layers indicate conditions either too arid to form deposits or so wet as to wash away sediments. Puzzlingly, some of these adaptive advances seem to precede, rather than follow after, the climatic extremes attained.

image

Figure 19.1

Global temperature trend through the Pliocene, Pleistocene and Holocene, noting with red arrows when unprecedented extremes were reached.

(image from giss.nasa.gov)

During these environmental transitions many species, genera and even families went terminally extinct. These included most of the Oligocene/Miocene giants among the herbivores, plus various sabretooth cats and hyenas among the carnivores. Meanwhile, the stem bovid generated the dazzling diversity of grazers plus browsers that we see still extant today. However, not all of these forms survived; several of the largest grazers faded out around the time of the most recent glacial maximum. The product of the primate lineage transforming australopithecine ape-men into modern humans endured, precariously at times, but ultimately became empowered to dominate the whole world.

What happened to the other recognised hominin species? Were they terminal side-branches, like the robust ape-men clearly were? Or chronospecies, connected through time? More fundamentally, were they really functional species, as defined biologically by genetic isolation? Were there indeed times when three or four species of hominins coexisted, side-by-side in the same places at the same time,2 violating ecological principles of niche separation? Looking beyond Africa, why did super-adaptable Homo erectus, the geographic counterpart of H. ergaster, eventually fade out in Asia, and why did Homo sapiens replace both the Neanderthals and Denisovans in Europe?

Preceding chapters have established an ecologically plausible scenario, consistent with the fragmented fossilised and cultural evidence that we have to build on. I have reported it using the names that have been given to putative species and genera, but perhaps this labelling is obscuring how these species connected in enduring lineages. What is the relationship between species turnover and lineage persistence?

Multi-regional and Reticulate Evolution

It is striking how evolutionary advances took place fairly synchronously in eastern and southern Africa, thousands of kilometres apart. Au. afarensis dates back to 3.8 Ma in the Afar region of north-eastern Ethiopia and Au. promethius, probably ancestral to Au. africanus, to 3.7 Ma in Sterkfontein Cave in South Africa (Chapter 17). The earliest fossils assigned to Homo ergaster from the Omo–Turkana Basin and a South African cave site both date to shortly after 2.0 Ma. H. heidelbergensis emerges around 0.8 Ma in Germany as well as in Africa. Middle Stone Age technology appears shortly before 200 ka across Africa from south to north. Finely crafted stone flakes typifying the Later Stone Age became established around 45 ka in both southern and eastern African sites. The synchronous spread of these evolutionary advances through Africa was undoubtedly enabled by the continuity of savanna grasslands straddling the equator, without mountain chains or river barriers blocking movements of animals and people.

The notion of synchronous multi-regional evolution was formerly advanced in support of beliefs that modern humans originated in Asia rather than in Africa. Supposedly, local adaptive innovations developed in parallel in different localities in response to similar selective pressures. The concept has recently been resurrected as an ‘African Multi-regional’ scenario.3 Nevertheless, it seems unlikely that the isolation of geographically separated populations endured sufficiently long in Africa, given the widely variable conditions in local regions.4,5 An alternative scenario has been characterised as reticulate evolution.6,7 It is envisaged that small local populations evolved independently but, rather than remaining isolated, were periodically reconnected to share genes, like a braided stream or reticulate web of linkages.3,5 Such a population structure could generate a polymorphic mixture of features distinguishing local populations, especially during transitional periods while the novel adaptations spread.

It is quite plausible that Paranthropus, grubbing for underground plant parts as a food staple, coexisted alongside early Homo, scavenging or hunting animals as well. These hominin forms seem too distinct in appearance and lifestyle to have exchanged genes. But it is not tenable ecologically for H. habilis and H. erectus and H. ergaster and H. rudolfensis to have coexisted in time and space as distinct species, as has been inferred.2 If they were indeed syntopic, as claimed, their distinguishing features are more likely to reflect polytypic variation within a broadly distributed population occupying the H. ergaster/erectus niche. Whether H. erectus geographically apart in Eurasia should be distinguished specifically from H. ergaster in Africa is moot. Representatives of Australopithecus and Paranthropus in eastern and southern Africa can acceptably be labelled as district species because we lack information about the geographic variation in between. H. naledi might have squeezed in alongside H. sapiens by being physically smaller and thus more selective among vegetation components exploited.

The morphological features used to distinguish the species represented in Figure 17.1 are based mostly on skulls, jaws and teeth. Taxa in the same grade eat the same sorts of food and share the same mode of locomotion.8 The form of the dentition can represent adaptive shifts in food resources exploited, notably between early Homo and Paranthropus, sufficient to enable coexistence. Brain size has multiple adaptive ramifications. However, other features such as the protrusion of the snout and the compaction of the face and brow ridges seem neutral ecologically, although potentially guiding mate recognition. It seems more likely that these cranial features drifted, as tends to happen in small temporarily isolated populations, without preventing the local populations from coalescing genetically at some later stage. As outlined in Chapter 14, subspecies of hartebeest are based on distinctions in horn shape and coat colour, without being judged sufficiently isolated to preclude later genetic merging. Geographically separated populations of baboons interbreed and thereby merge gene pools, whether distinguished as species or subspecies.9

Adversity Versus Variability Selection

Potts10,11 related the advances shown in the lineage leading to Homo to the need to adapt to widening environmental fluctuations in temperature and aridity, which he labelled ‘variability selection’. He proposed that hominins responded to the highly variable climatic conditions in African savannas by enhancing their adaptive versatility, for example by broadening their diet to cope with both meat and plant matter. However, it is the harshness of the dry extremes that would have formed the selective filter, more than the mesic upswings. Typically, fitness curves flatten towards benign conditions, but fall increasingly steeply with intensifying insufficiency of food supplies, producing ‘Jensen’s inequality’ (Figure 19.2). Hence variation in rainfall or other conditions above the mean has less effect on resource gains, and hence fitness, than variation below the mean. The maximum rate of population growth is constrained by life-history features when conditions are benign (Chapter 12), but rates of population decline in lean times are in free fall. When rates of recovery fail to compensate for the crashes, populations go into decline, ratcheting downwards towards eventual extinction with each successive knock of adversity.12 I interpret this pattern as adversity selection.

image

Figure 19.2

Jensen’s inequality, showing how variation in rainfall above the mean makes less difference to resource gains, and hence fitness, than equivalent variation below the mean, because of the curvilinearly saturating trend.

(taken from Owen-Smith (2002) Adaptive Herbivore Ecology: From Resources to Populations in Variable Environments)

There is increasing recognition of how rapidly adaptive shifts in local populations can occur in response to environmental changes, undermining notions of gradually progressive advancements.13 Adaptive shifts can be especially abrupt during climatic extremes, which impose strong selection for survival-enhancing features of morphology or physiology. The beaks of finches in the Galapagos changed size and shape within a few decades in response to climatic extremes affecting characteristics of the seeds that they sought.14 Guppies altered their colour patterns within less than a decade in response to the addition or removal of predators.15 The morphological, physiological and behavioural features of hominins could have shifted within several centuries or a few millennia under strong selection during times of unprecedented adversity.

For hominins, the extremes of aridity reached during glacial advances imposed the selective filter. Recall the associations in time between the major adaptive transitions among hominins noted above and the surges towards unprecedented extremes in low temperatures manifested globally around these times (Figure 19.1). In African savannas it was not temperature that formed the prime selective filter, but rather the extreme shortages of plant resources that developed during the dry season at times when global conditions were coldest. Moreover, regional rainfall was not necessarily coupled solely with global temperature changes. There were additional influences from tectonic upheavals affecting the locations of rain shadows and diverting monsoon winds, while the ITCZ also shifted its seasonal positioning, affecting local rainfall (Chapters 1 and 2).

During severe droughts, large herbivore populations can crash to less than half of their prior abundances.16,17,18 If food runs out before the end of the dry season, mortality losses as great as 80–90 percent of local populations can ensue.19,20 This is powerful selection for any attribute conferring greater chances of being among the survivors. Such extremes of adversity probably recurred multiple times around each glacial maximum. Die-offs recorded during the currently prevailing interglacial interlude do not adequately convey the devastation of the extremes of aridity attained in the past, potentially resulting in local or even continental extinctions. We do know that the two most recent glacial advances were both associated with multiple extinctions among large grazers. These included Reck’s elephant, gorgops hippo, giant zebra, long-horned buffalo, two blesbok, giant gelada and giant warthog in eastern Africa, and long-horned buffalo, giant wildebeest, a blesbok, a large zebra and two species of springbok in southern Africa. Amazingly, the hominin lineage leading to us made it through every episode of adversity, albeit precariously, although not unchanged.

I conceptualised the dynamic niche as a trajectory through hyperspace to explain how herbivores cope with varying food availability over the seasonal cycle (Figure 19.3).21 Population persistence depends crucially on how animals find something to eat during savanna dry seasons, while avoiding being eaten by carnivores. Large herbivores achieve this by shifting diets and habitats occupied in functionally distinctive ways, exploiting environmental heterogeneity (Figure 19.3). Survival is multiplicative; if at any stage survival prospects become zero, that is it – local extinction results. For global extinctions, populations must run out of resources, or become subject to non-sustainable levels of predation, everywhere across the species distribution range, a much more restrictive requirement. The wider a species is distributed, the less the chance of terminal extinction. The Africa-wide distribution of the lineage labelled Homo dependent on its dietary flexibility surely contributed to its lineage endurance. How it escaped being eliminated by predation, being neither fierce nor fast, remains less apparent.

image

Figure 19.3

Dynamic niche concept, illustrating how herbivores cope with seasonal variation in environmental conditions through exploiting spatial heterogeneity. Five habitat units are distinguished discretely by the seasonally varying resource gains that they potentially convey, represented by the height of the blocks. Herbivores shift their habitat and resource selection to enable survival through the seasonal cycle. Resource gains could be translated into fitness by considering additionally reductions in survival brought about by predation in each habitat type.

(taken from Owen-Smith (2002) Adaptive Herbivore Ecology: From Resources to Populations in Variable Environments)

Why Only in Africa?

The reasons why the crucial evolutionary transformations from forest-dwelling primates to savanna-inhabiting humans could only have occurred in Africa did indeed originate from geo-tectonics. The uplift of Africa and subsequent rifting and volcanics established the physical cradle. It produced unusually low rainfall for the tropics coupled with soils that remained comparatively fertile, at least for nourishing large herbivores. Had Africa been mostly low-lying, it would have become largely a degraded semi-desert like most of Australia. Had the uplift taken place in the west, tropical regions of Africa would have been as moist and infertile as South America, thronged with huge mega-grazers rather than a rich assemblage of medium–large ruminants. Tropical Asia is mostly too low and wet for savanna vegetation to be extensive and lacks an abundant grazing fauna. An intellectual debt must be acknowledged to the geologists who saw that tectonics was crucially responsible for promoting human evolution.22 I have explored the connections in this book and shown that their perception is justified.

The crucial feature of Africa’s climates is the wide prevalence of seasonal dryness. This underlies the spatial predominance of savanna vegetation, with grasses coexisting between and beneath trees. Ruminants radiating during the Miocene adapted especially to digest the fibrous C4 grasses during the dry seasons. This capability enabled these grazers to attain vastly greater abundances than browsers having their food base restricted to the few leaves remaining within reach on trees during dry seasons. A diet of dry grass made the grazers dependent on access to surface water, concentrating their numbers within reach of perennial water sources during the dry months. This opened opportunities for savanna-dwelling ape-men to incorporate animal flesh into their diet, compensating for the lack of plant food and the effort involved in extracting what remained underground during the dry season. The dietary shift set up an evolutionary train of adaptations for endurance locomotion, including bared skin for efficient sweating and stone tools for extracting marrow and meat.

South America’s large grazers were mostly too big to be exploited sustainably for flesh in tropical climates where meat soon rots. Australia and tropical Asia were both deficient in grazers. Nowhere outside of Africa were large herbivores sufficiently abundant to nurture the seasonal dependency of comparatively puny primates on scrounging from carnivore kills or running down their own prey.

The size structure of Africa’s large herbivore fauna was also crucially important.23 Carcasses of small antelope get consumed completely by their mammalian carnivore killers. Those of megaherbivores that have died remain attended by carnivores until the meat turns putrid, while the huge marrow-containing bones are not easily transported to safer sites for processing, let alone their skulls. The unique feature of Africa’s large herbivore fauna is the abundance and diversity of medium–large ruminants, weighing 50–500 kg, which was established by 5 Ma. The dominance of the grazers among them was promoted specifically by the prevalence of dry/eutrophic savannas, or ‘sweetveld’, with relatively palatable grasses, a savanna subdivision not recognised in other continents. Tools developed by hominins to extract and pulverise tough plants became deployed to break open the bones and scrape flesh off the ungulate carcasses abandoned by the big fierce killers, exploiting a time window when large carnivores were mostly inactive.

However, simply adding meat to the diet was not enough. It needed to be obtained reliably, even after herbivore populations had crashed in very dry years. Ingenuity was required to locate where herbivores remained and dispatch them reliably using increasingly effective tools, like poison-tipped arrows. Brains expanded to accommodate this anticipatory planning and contributed to cultural evolution and language development. The greater cerebral competence needed to cope with climatic uncertainty and periodic extremes of aridity exerted strong selection for adaptations that increased survival odds through the crunch periods. Several of the large grazers that had taken form during the Pliocene and Pleistocene fell by the wayside as glacial conditions reached ever greater levels of cold and aridity. Adaptive shifts appeared abruptly, seemingly synchronised with climatic extremes. The ecologically adept hominin survivors dispersed from their local habitats to spread their crucial adaptations through Africa and beyond. After some modern humans acquired domestic livestock from across the Mediterranean Sea and became herders, savanna grasslands enticed them southward amid the wealth of wild herbivores that they no longer needed to hunt, except on ceremonial occasions or during times after livestock herds crashed during droughts.

Space to move, by both people and wildlife, is currently being suppressed as modern humans saturate the Earth, from the equator to the poles. What are the prospects for survival of the faunal diversity that contributed so fundamentally to our origins? This is the topic that I will address in the concluding chapter.

SUGGESTED FURTHER READING

Potts, R. (2012) Environmental and behavioural evidence pertaining to the evolution of early Homo. Current Anthropology 53(Suppl. 6):S299–S317.

Scerri, EML., et al. (2018) Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology and Evolution 33:582–592.

Stringer, C. (2016) The origin and evolution of Homo sapiens. Philosophical Transactions of the Royal Society B: Biological Sciences 371:20150237

REFERENCES

1.Wood, BK; Boyle, E. (2016) Hominin taxic diversity: fact or fantasy? American Journal of Physical Anthropology 159:37–78.

2.Bobe, R; Carvalho, S. (2019) Hominin diversity and high environmental variability in the Okote Member, Koobi Fora Formation, Kenya. Journal of Human Evolution 126:91–105.

3.Stringer, C. (2016) The origin and evolution of Homo sapiens. Philosophical Transactions of the Royal Society B: Biological Sciences 371:20150237.

4.Klein, RG. (2019) Population structure and the evolution of Homo sapiens in Africa. Evolutionary Anthropology: Issues, News, and Reviews 28:179–188.

5.Scerri, EML, et al. (2018) Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology & Evolution 33:582–594.

6.Arnold, ML. (2009) Reticulate Evolution and Humans: Origins and Ecology. Oxford University Press, Oxford.

7.Winder, IC; Winder, NP. (2014) Reticulate evolution and the human past: an anthropological perspective. Annals of Human Biology 41:300–311.

8.Wood, B. (2010) Reconstructing human evolution: achievements, challenges, and opportunities. Proceedings of the National Academy of Sciences of the United States of America 107:8902–8909.

9.Fischer, J, et al. (2019) The natural history of model organisms: insights into the evolution of social systems and species from baboon studies. Elife 8:e50989.

10.Potts, R. (1998) Environmental hypotheses of hominin evolution. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists 107:93–136.

11.Potts, R. (2013) Hominin evolution in settings of strong environmental variability. Quaternary Science Reviews 73:1–13.

12.Ogutu, JO; Owen‐Smith, N. (2003) ENSO, rainfall and temperature influences on extreme population declines among African savanna ungulates. Ecology Letters 6:412–419.

13.Reznick, DN, et al. (2019) From low to high gear: there has been a paradigm shift in our understanding of evolution. Ecology Letters 22:233–244.

14.Grant, PR, et al. (2017) Evolution caused by extreme events. Philosophical Transactions of the Royal Society B: Biological Sciences 372:20160146.

15.Reznick, DN, et al. (1997) Evaluation of the rate of evolution in natural populations of guppies (Poecilia reticulata). Science 275:1934–1937.

16.Young, TP. (1994) Natural die‐offs of large mammals: implications for conservation. Conservation Biology 8:410–418.

17.Dublin, HT; Ogutu, JO. (2015) Population regulation of African buffalo in the Mara–Serengeti ecosystem. Wildlife Research 42:382–393.

18.Smit, IPJ; Bond, WJ. (2020) Observations on the natural history of a savanna drought. African Journal of Range & Forage Science 37:119–136.

19.Spinage, CA; Matlhare, JM. (1992) Is the Kalahari cornucopia fact or fiction? A predictive model. Journal of Applied Ecology 29:605–610.

20.Walker, BH, et al. (1987) To cull or not to cull: lessons from a southern African drought. Journal of Applied Ecology 24:381–401.

21.Owen-Smith, RN. (2002) Adaptive Herbivore Ecology: From Resources to Populations in Variable Environments. Cambridge University Press, Cambridge.

22.Gani, MR; Gani, NDS. (2008) Tectonic hypotheses of human evolution. Geotimes 53:34–39.

23.Owen‐Smith, N. (2013) Contrasts in the large herbivore faunas of the southern continents in the late Pleistocene and the ecological implications for human origins. Journal of Biogeography 40:1215–1224.

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