Creationism in Crisis - How Interspecies Rivalry Gave Rise To Multiple Hominin Species

A cast of the skull of Homo Heidelbergensis

Credit: The Duckworth Laboratory

Australopithecus sediba

Interspecies competition led to even more forms of ancient human – defying evolutionary trends in vertebrates | University of Cambridge

The gulf between science and creationism continues to widen with the publication of an open access research paper in Nature Ecology & Evolution in which the two authors argue that early hominins speciates as a result of inter-species competition, but not like most vertebrates through competition for niches, but because the evolution of technology enabled species to evolve quickly into new niches with what amounts to memetic evolution, i.e., evolution of cultures which, in an intelligent species can occur much more quickly than the slow, genetic evolution in other species.

In this respect the pattern of early hominin evolution was more like that of beetles evolving on an island.

Creationists, by contrast, as still stuck desperately looking for evidence that all humans are descended from a single pair of humans who were magically made from dirt just a few thousand years ago.

The researchers, Laura A. van Holstein and Robert A. Foley of the Leverhulme Centre for Human Evolutionary Studies, Department of Archaeology, University of Cambridge, Cambridge, UK, explain their research in a Cambridge University new release:

Competition between species played a major role in the rise and fall of hominins, and produced a “bizarre” evolutionary pattern for the Homo lineage.

This is almost unparalleled in evolutionary science

Laura van Holstein, lead author
Leverhulme Centre for Human Evolutionary Studies
Department of Archaeology
University of Cambridge, Cambridge, UK

Climate has long been held responsible for the emergence and extinction of hominin species. In most vertebrates, however, interspecies competition is known to play an important role.

Now, research shows for the first time that competition was fundamental to 'speciation' – the rate at which new species emerge – across five million years of hominin evolution.

The study, published today in Nature Ecology & Evolution, also suggests that the species formation pattern of our own lineage was closer to island-dwelling beetles than other mammals.

We have been ignoring the way competition between species has shaped our own evolutionary tree. The effect of climate on hominin species is only part of the story.

Laura van Holstein.

In other vertebrates, species form to fill ecological “niches” says van Holstein. Take Darwin’s finches: some evolved large beaks for nut-cracking, while others evolved small beaks for feeding on certain insects. When each resource niche gets filled, competition kicks in, so no new finches emerge and extinctions take over. Van Holstein used Bayesian modelling and phylogenetic analyses to show that, like other vertebrates, most hominin species formed when competition for resources or space were low.

The pattern we see across many early hominins is similar to all other mammals. Speciation rates increase and then flatline, at which point extinction rates start to increase. This suggests that interspecies competition was a major evolutionary factor.

Laura van Holstein.

However, when van Holstein analysed our own group, Homo, the findings were 'bizarre'. For the Homo lineage that led to modern humans, evolutionary patterns suggest that competition between species actually resulted in the appearance of even more new species – a complete reversal of the trend seen in almost all other vertebrates.

The more species of Homo there were, the higher the rate of speciation. So when those niches got filled, something drove even more species to emerge. This is almost unparalleled in evolutionary science.

Laura van Holstein.

The closest comparison she could find was in beetle species that live on islands, where contained ecosystems can produce unusual evolutionary trends.

The patterns of evolution we see across species of Homo that led directly to modern humans is closer to those of island-dwelling beetles than other primates, or even any other mammal.

Laura van Holstein.

Recent decades have seen the discovery of several new hominin species, from Australopithecus sediba to Homo floresiensis. Van Holstein created a new database of 'occurrences' in the hominin fossil record: each time an example of a species was found and dated, around 385 in total. Fossils can be an unreliable measure of species’ lifetimes. “The earliest fossil we find will not be the earliest members of a species,” said van Holstein.

How well an organism fossilises depends on geology, and on climatic conditions: whether it is hot or dry or damp. With research efforts concentrated in certain parts of the world, and we might well have missed younger or older fossils of a species as a result.

Laura van Holstein.

Van Holstein used data modelling to address this problem, and factor in likely numbers of each species at the beginning and end of their existence, as well as environmental factors on fossilisation, to generate new start and end dates for most known hominin species (17 in total).

She found that some species thought to have evolved through 'anagenesis' – when one slowly turns into another, but lineage doesn’t split – may have actually 'budded': when a new species branches off from an existing one.

Australopithecus afarensis ('Lucy') - artist's impression.

For example, the hominin species Australopithecus afarensis was believed to have speciated via anagenesis from Australopithecus anamensis. However, the new data modelling suggests they overlapped by around half a million years.

This meant that several more hominin species than previously assumed were co-existing, and so possibly competing.

While early species of hominins, such as Paranthropus, probably evolved physiologically to expand their niche – adapting teeth to exploit new types of food, for example – the driver of the very different pattern in our own genus Homo may well have been technology.

Adoption of stone tools or fire, or intensive hunting techniques, are extremely flexible behaviours. A species that can harness them can quickly carve out new niches, and doesn’t have to survive vast tracts of time while evolving new body plans.

Laura van Holstein.

She argues that an ability to use technology to generalise, and rapidly go beyond ecological niches that force other species to compete for habitat and resources, may be behind the exponential increase in the number of Homo species detected by the latest study.

But it also led to Homo sapiens – the ultimate generalisers. And competition with an extremely flexible generalist in almost every ecological niche may be what contributed to the extinction of all other Homo species.

These results show that, although it has been conventionally ignored, competition played an important role in human evolution overall. Perhaps most interestingly, in our own genus it played a role unlike that across any other vertebrate lineage known so far.

Laura van Holstein.

Abstract

The search for drivers of hominin speciation and extinction has tended to focus on the impact of climate change. Far less attention has been paid to the role of interspecific competition. However, research across vertebrates more broadly has shown that both processes are often correlated with species diversity, suggesting an important role for interspecific competition. Here we ask whether hominin speciation and extinction conform to the expected patterns of negative and positive diversity dependence, respectively. We estimate speciation and extinction rates from fossil occurrence data with preservation variability priors in a validated Bayesian framework and test whether these rates are correlated with species diversity. We supplement these analyses with calculations of speciation rate across a phylogeny, again testing whether these are correlated with diversity. Our results are consistent with clade-wide diversity limits that governed speciation in hominins overall but that were not quite reached by the Australopithecus and Paranthropus subclade before its extinction. Extinction was not correlated with species diversity within the Australopithecus and Paranthropus subclade or within hominins overall; this is concordant with climate playing a greater part in hominin extinction than speciation. By contrast, Homo is characterized by positively diversity-dependent speciation and negatively diversity-dependent extinction—both exceedingly rare patterns across all forms of life. The genus Homo expands the set of reported associations between diversity and macroevolution in vertebrates, underscoring that the relationship between diversity and macroevolution is complex. These results indicate an important, previously underappreciated and comparatively unusual role of biotic interactions in Homo macroevolution, and speciation in particular. The unusual and unexpected patterns of diversity dependence in Homo speciation and extinction may be a consequence of repeated Homo range expansions driven by interspecific competition and made possible by recurrent innovations in ecological strategies. Exploring how hominin macroevolution fits into the general vertebrate macroevolutionary landscape has the potential to offer new perspectives on longstanding questions in vertebrate evolution and shed new light on evolutionary processes within our own lineage.

Main

The diversification of a lineage is the net output of speciation minus extinction. A theme that runs through much research into human evolution is whether the determinants of hominin diversification conform to or diverge from those seen in other taxa. At one extreme lie ideas such as Wolpoff’s ‘single species hypothesis’¹, which suggested that there can be no speciation in the hominin lineage, as its niche is ‘culture’. Culture, in Wolpoff’s view, is uniquely human and prevents boundaries between populations from occurring; hence, speciation was prohibited in hominins, but not in other clades. At the other extreme are interpretations that emphasize commonalities between patterns of hominin speciation and extinction and those of other clades². Within this group, research interest has primarily been devoted to examining the role of climate in shaping hominin diversification^2,3,4,5,6,7. What has received far less attention as a potential driver of hominin diversification than climate, however, is competition.

Competition occurs across taxonomic scales, from interindividual competition within populations⁸ to intergroup competition within species⁹ and interspecific competition¹⁰. Competition at each of these levels has been shown to act as an important driver of evolution at equal or higher scales^11,12,13. Here we focus on interspecific competition for niches (hereafter ‘competition’) and its consequences above the species level. Although the concept of ‘niche’ has only rarely been formally defined in previous work on diversity-dependent speciation^14,15,16, its implicit definition in previous work is that of a Hutchinsonian ecological niche¹⁷—an n-dimensional hypervolume describing all environmental resources and conditions required for species persistence. We adopt this conventional definition throughout this paper. The consequences of competition can include three processes: speciation, extinction and morphological change through, for example, character displacement^13,18. There is some indirect evidence that competition resulted in morphological evolution in our lineage: competition between Homo and Paranthropus in East Africa probably led to character displacement in the mandibular premolar morphology of these two groups¹⁹. However, much less work has been devoted to exploring the effects of competition on hominin speciation or extinction.

Ecological competition with large carnivores is thought to have exerted a strong effect on hominin ranging patterns^20,21, hunting behaviour²² and, of particular interest at a macroevolutionary scale, geographic dispersals^23,24. Although it is unknown whether competition with large carnivores had direct effects on hominin speciation or extinction, the link between dispersals and these macroevolutionary processes is well established²⁵. Compared with competition between hominins and non-hominins, the dynamics and effects of competition between hominin species have received comparatively little attention. Although competition may have contributed to a pulse of hominin extinction around 1.5 million years ago²⁶, and some recent reviews have used evidence for hominin sympatry in East and South Africa to suggest the possibility of competition^27,28, an explicit investigation of the extent to which competition drove hominin diversification is lacking.

Competition has probably had a major role in animal diversification, however, leaving signals in correlations between species diversity, on the one hand, and speciation and extinction on the other^{14,16,29,30,31}. Speciation can be both positively or negatively diversity dependent or occur independently of a clade’s own diversity. Under positive diversity dependence, speciation rates rise as a function of the novel evolutionary opportunities and interactions created by other species³². This pattern is exceptionally rare among all forms of life, however, having been reported only in island-dwelling beetles³³, plants and arthropods³⁴, and this latter case is contentious^35,36. Instead, if a relationship exists between vertebrate speciation and diversity, this is usually negative^14,16,30,31. There are two processes by which speciation may be negatively controlled by diversity: competition for (1) niche space, or (2) geographic space15. In both, speciation is regulated by bounded ecological opportunities. In classical Darwinian diversity dependence^16,37, speciation into a niche occupied by a closely related species is prohibited, producing a negative relationship. At a higher level of taxonomic organization, models of asymptotic diversity predict slowdowns in speciation as a finite number of niches within an adaptive grade, or a finite number of ranges within bounded space, become occupied by closely related species as a clade grows^25,38. However, findings of diversity-independent speciation in some clades has led to intense debate about whether negative diversity dependence is universal across vertebrates; the same is true for the related question of whether absolute limits to niches or geographic ranges even exist^39,40.

The relationship between extinction and diversity has received less explicit empirical attention than that between speciation and diversity. However, when a relationship is reported, extinction is typically positively diversity dependent^41,42. These patterns align with expectations based on theory. Under Darwinian diversity dependence, competition between ecologically similar species should result in extinction of outcompeted species³⁷ even in the absence of absolute limits to species diversity. Models of asymptotic diversity^16,43 predict increased rates of extinction as species diversity approaches an explicitly predicted diversity limit. Asymptotic diversity dynamics have been reported for multiple vertebrate clades^44,45, although other studies have suggested that these trends are unclear among terrestrial vertebrates³². As is the case for speciation, then, there is some empirical evidence for a typical direction of the relationship between diversity and extinction—in this case, positive—but the universality of this pattern among vertebrates, too, remains an open question.

Hominin evolution is represented by a well-studied and rich fossil record and occurs across temporal and spatial scales that sit squarely at the expected intersection of climatic and competitive processes46. Therefore, exploring how hominin macroevolution fits into the general vertebrate macroevolutionary landscape has the potential to offer new perspectives on longstanding questions in vertebrate evolution, as well as addressing the comparative dearth of explicit research on diversity-dependent macroevolution in the hominin lineage.

Here we ask whether hominins also follow the pattern of diversity-dependent diversification that characterizes many other vertebrate clades. More specifically, we ask: were hominins characterized by negative diversity-dependent speciation and positive diversity-dependent extinction?

At which taxonomic level should these patterns be expected? Negative diversity-dependent speciation and positive diversity-dependent extinction at the level of the hominin clade as a whole would imply either that hominins were characterized by species’ inabilities to diverge ecologically from each other, as in Darwinian diversity dependence—and in an extension of Wolpoff’s ‘culture’ argument—or that hominins occupy a bounded set of niches in broader ecological context, as in asymptotic diversity dependence. One possibility is that hominin diversification is not diversity dependent, either because hominin speciation and extinction are purely climate-driven and not determined by diversity-mediated competitive dynamics^2,3,4,5,6,7, or because the lineage was characterized by consistent ecological divergence, or because a limit to species diversity did not exist or was not reached. A second possibility is that hominins, overall, conform to the expected patterns. This would indicate a powerful and underappreciated role for interspecific competition in hominin evolution.

However, Darwinian diversity dependence^16,37 predicts stronger signals of diversity-dependent dynamics within and not across adaptive grades^15,16,37, as species within adaptive grades should be more ecologically similar to each other. Given that there is strong support for Homo having occupied an adaptive grade distinct from earlier hominins^47,48, we contrast the patterns found between Homo and Plio-Pleistocene non-Homo species (Australopithecus and Paranthropus). In addition to the two possible patterns described above, this comparison presents a third possibility: conflicting patterns between adaptive grades. Such a pattern will have resonance with the major issue of how far hominin evolution conforms to general evolutionary patterns, and why it might diverge.

Fig. 1: Speciation and extinction dates and phylogeny used in subsequent analyses.

a, Species lifespans, comprising the time between speciation and extinction dates based on three datasets. Orange: published fossil FADs and LADs estimated without taking fossil preservation into account. Light blue: speciation and extinction dates estimated in a Bayesian framework incorporating time-based variability in fossil preservation rates. Dark blue: speciation and extinction dates estimated in a Bayesian framework incorporating within-lineage variability in fossil preservation rates. Note that these taxa are those the published dates and our new database have in common; actual analyses incorporated Homo ergaster in the no-preservation-prior dataset and Homo rudolfensis in the preservation prior datasets. Homo erectus s.l. refers to Homo erectus sensu lato. b, The Parins-Fukuchi et al.⁸⁷ phylogeny used in this study, with species coloured by taxonomic grouping (yellow: Homo; green: non-Homo).

van Holstein, L.A., Foley, R.A.
Diversity-dependent speciation and extinction in hominins.
Nat Ecol Evol (2024). https://doi.org/10.1038/s41559-024-02390-z

Copyright: © 2024 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

The disturbing thing for creationists in this is that the scientists were not debating whether the Theory of Evolution is better than the creationists superstition at explaining the facts but exactly how this fits in with the TOE and what it tells us of the mechanisms at play in the evolution of our hominin ancestors.

The childish notion that humans were magicked up from dirt just a few thousand years ago without ancestors is not even a consideration for palaeoanthropologists.

The interesting thing is the degree to which gene-meme co-evolution played a part on the evolution of modern humans and indeed the plethora of co-existing, and sometime interbreeding, hominin species that seem to have been the norm until about 40,000 years ago, about which, of course, the authors of Genesis were completely ignorant.

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