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A Disappointing Decade for the Study of Human Evolution

Evolution News

The 2010s was a bad decade for the study of human evolution. Smithsonian Magazine recently published an article titled “These are the Decade’s Biggest Discoveries in Human Evolution.” It opens by saying:

Human evolution is one of the most vibrant areas of scientific investigation. In the past decade we’ve seen many discoveries that add to our understanding of our origins. To mark the 10th anniversary of the Smithsonian’s “David H. Koch Hall of Human Origins,” here are some of the biggest discoveries in human evolution from the last 10 years.

What are the big discoveries of the decade? Did they reveal new and compelling evidence that humans evolved from lower primates? Some of these big discoveries actually turn out to be instances where the evidence for human evolution weakened, and the rest amount to slight revisions of previously held theories that don’t say much about the core tenets of paleoanthropology.

Ancient DNA

Smithsonian’s first big science breakthrough is the discovery of ancient DNA. Indeed, this is an exciting development for the burgeoning field of paleogenetics — and it has shown that there were various sublineages of very modern human-like groups such as the Denisovans that have existed in the past million years. Though a novel and intriguing form of evidence, at present ancient DNA is essentially silent on two crucial tenets of evolutionary paleoanthropology: (1) the hypothesis that modern humans are descended from decidedly non-human, subhuman, or otherwise primitive species, and (2) the claim that these newly revealed “species of early humans” were substantially different from us. Yes we have Denisovan DNA, but at present for all we know the Denisovans were as human as we are.

The second big discovery of the last decade was Homo naledi, which is also exciting because it represents a humungous cache of hominid fossils that adds a lot to our knowledge of the fossil record. Initially, news reports called Homo naledi a “human ancestor.” However in 2017 it was found that this species is only a few hundred thousand years old — 10 times too young to be considered as one of our evolutionary ancestors. This was a major bust for proponents of human evolution, as we reported here.

Bring Out Your Dead

Another major claim about Homo naledi was that it buried its dead — the implication that this small-brained species was beginning to show glimmers of human-like behavior and intelligence. But there are major doubts about this — see here for a review. Homo naledi was a spectacular fossil discovery, but the evidence that it was a small-brained but intelligent species that was ancestral to humans simply did not pan out.

In fact, if there’s any major theme of paleoanthropology in the 2010s, it’s that new discoveries and analyses of fossil ages conflicted with the preferred evolutionary story.

An Unexpectedly Young Age

Thus, the third big find of the decade listed by Smithsonian is the discovery of a near-complete skull of Australopithecus anamensis last year, dated to about 3.8 million years old. Günter Bechly wrote an excellent review of this fossil find here at Evolution News. He pointed out that this skull allowed scientists to understand, for the first time, what the species A. anamensis actually looked like. However, its unexpectedly young age meant that it overlapped in time with its supposed descendants, the species A. afarensis. Science Daily quoted one of the scientists involved with the find as saying, “We used to think that A. anamensis gradually turned into A. afarensis over time.” But because of the age of the fossil, they no longer think this is the case. Bechly explained:

Such anagenetic evolution by gradual species-to-species transitions (without branching events) is actually predicted by Darwin’s theory. Therefore, we should expect to find some fossil evidence for this crucial process. But such evidence turned out to be elusive (see below), and the case of the supposed transition from A. anamensis to A. afarensis was “one of the strongest cases for anagenesis in the fossil record” (Melillo quoted in Marshall 2019, Kimble et al. 2006, Haile-Selassie 2010). This strongest case has now evaporated, and it was not only the strongest case but also the last case…

To retell the story, in 2006 there was a discovery of A. anamensisfossils that predated A. afarensis. This generated quite a stir. Paleoanthropologists decided it showed a direct ancestor-descendant relationship between the two species. At that time Tim White and others wrote in Nature: “The earliest described Australopithecus species is Au. anamensis, the probable chronospecies ancestor of Au. afarensis.” The story was picked up by the media and touted as really powerful evidence for evolution. For example, the Associated Press reported:

The latest fossil unearthed from a human ancestral hot spot in Africa allows scientists to link together the most complete chain of human evolution so far. … “We just found the chain of evolution, the continuity through time,” study co-author and Ethiopian anthropologist Berhane Asfaw said in a phone interview from Addis Ababa. “One form evolved to another. This is evidence of evolution in one place through time.

This presumed example of the gradual evolution of a direct ancestor-descendant relationship has been very important not just to media promoters of Darwinism, but also to paleoanthropologists. That is because it is so rare that they think they have an example of such a thing. In fact, a major 2015 review of hominid evolution, by Bernard Wood and Mark Grabowski, in the book Macroevolution: Explanation, Interpretation and Evidence admitted that bona-fide examples of ancestor-descendant sequences in hominin fossils are extremely rare:

Most hominin taxa, particularly early hominins, have no obvious ancestors, and in most cases, ancestor-descendent sequences (fossil time series) cannot be reliably constructed — two possible exceptions are mentioned below. 

And guess what one of their two best examples was? It was A. anamensis gradually evolving directly into A. afarensis:

At one time, or another, every early hominin discussed above has been presented as “the” ancestor of later hominins, but in our opinion, only two pairs of taxa, Au. anamensis and Au. afarensis (Kimbel et al. 2006), and P. aethiopicus and P. boisei (Wood and Schroer 2013), are plausible examples of ancestor/descendant relationships (i.e., are examples of anagenesis). In the case of the former pair, Au. anamensis and Au. afarensis are most likely time-successive taxa within a single lineage with the Laetoli hypodigm of the former taxon intermediate between Au. anamensis and the Hadar hypodigm of Au. afarensis. This hypothesis has been given support by the discovery of fossil evidence from Woranso-Mille in Ethiopia that is both temporally and morphologically intermediate between Au. anamensis and Au. afarensis (Haile-Selassie et al. 2010).

However, this example of an “ancestor-descendant sequence” can no longer be used because the A. anamensis skull discovered last year dates to 3.8 Ma, which is 100,000 years after the appearance of fossils of A. afarensis. A gradualistic transition is no longer feasible.

Why Does This Matter? 

Well, as we all know evolutionary biologists prefer to show gradualistic direct evolution rather than punc eq. That’s because the latter implies rapid evolution without fossil transitions whereas gradualistic evolution implies things evolve slowly, just as Darwin predicted. White et al. (2006)’s paper touted the evolution of A. anamensis into A. afarensisas as a potentially great example of a form of “gradualism” or “anagenesis” over “punctuated equilibrium”:

Gould suspected that “punctuated gradualism” was rare. In contrast, punctuated equilibrium (with speciation by “budding cladogenesis”) is thought to be more common, but demonstrating it requires the verified contemporaneity and persistence of both the ancestral and daughter species. As Gould noted, “We can distinguish the punctuations of rapid anagenesis from those of branching speciation by invoking the eminently testable criterion of ancestral survival following the origin of a descendant species. If the ancestor survives, then the new species has arisen by branching. If the ancestor does not survive, then we must count the case either as indecisive, or as good evidence for rapid anagenesis — but, in any instance, not as evidence for punctuated equilibrium.” (p. 795). These requirements have rarely been met among fossil hominids.

For the origin of Australopithecus, phyletic evolution with a burst of rapid directional change during the 200,000-yr period between 4.4 and 4.2Myr ago remains plausible given the geographic, temporal and morphological relationships of Ar. ramidus and Au. anamensis and our understanding of primate dental anatomy and development. Indeed, given the available evidence, the origin of Australopithecus could well turn out to be a case of “punctuated gradualism” or “punctuated anagenesis” rather than rectangular evolution sensu Stanley. 

So they admit that the requirements of gradual evolution “have rarely been met among fossil hominids” but White et al.’s 2006 paper predicted this was a case of gradualism. They note that this is easy to test: “If the ancestor survives, then the new species has arisen by branching” — i.e., by punctuated equilibrium.

Well, this 2019 skull discovery shows the presumed ancestor survived, because A. anamensis was apparently still around at 3.8 Ma, 100,000 years after its supposed descendant A. afarensis was already in existence. As the 2019 Nature paper about the discovery states:

We further demonstrate that A. anamensis and Australopithecus afarensis differ more than previously recognized and that these two species overlapped for at least 100,000 years—contradicting the widely accepted hypothesis of anagenesis. … Most importantly, MRD shows that despite the widely accepted hypothesis of anagenesis, A. afarensis did not appear as a result of phyletic transformation. It also shows that at least two related hominin species co-existed in eastern Africa around 3.8 Myr ago, further lending support to mid-Pliocene hominin diversity.”

Even though they still think there’s a possible ancestor-descendent relationship between anamensis and afarensis, we must now cross this example off the list of “phyletic gradualism” or what Wood and Grabowski called clear-cut “examples of ancestor/descendant relationships (i.e., are examples of anagenesis).” Or, as Science Daily put it:

This temporal overlap challenges the widely-accepted idea of a linear transition between these two early human ancestors. … We used to think that A. anamensisgradually turned into A. afarensis over time. We still think that these two species had an ancestor-descendent relationship, but this new discovery suggests that the two species were actually living together in the Afar for quite some time. [Emphasis added.]

A Third Example

Incidentally, in addition to A. anamensisiA. afarensis, and Homo naledi, there’s a third example from the 2010s where the ages of hominid fossils posed problems for the standard evolutionary model. We also reported last year that a study of Australopithecus sediba and the hominin fossil record published in Science Advances concluded that there is less than a 0.1 percent chance that A. sediba could be a human ancestor. That’s because it postdates the appearance of its would-be descendants in the genus Homo by about 100,000 years.  Somehow that study failed to make Smithsonian’s list of top discoveries of the decade.

There are three additional big discoveries of the decade listed by Smithsonian. These pertain to slight revisions of ages that don’t greatly affect the overall evidence for human evolution: (1) that our species, Homo sapiens, now dates back to 300,000 years ago, (2) that stone tools now date back to 3.3 million year ago, and (3) that modern humans are now thought to have gone “out of Africa” earlier, due to a jawbone in Israel that was dated to 174,000-185,000 years old. The first discovery is, again, just a slight modification of previously held dates, and the second age may very well be true although precise ages of tools are notoriously difficult to determine. As for the third discovery, Smithsonian Magazine puts it this way:

[I]n 2018, researchers announced the discovery of an upper jaw in Israel that looked like that of our own species, Homo sapiens. The jaw ended up being 174,000-185,000 years old. This discovery — along with others from China and Greece—suggest that Homo sapiens wandered short-term into Eurasia well before the worldwide migration that began 70,000 years ago.

Commenting on 2018, which was an “annus horribilis” (“terrible year”) for paleoanthropology, Günter Bechly wrote:

The authors of this new study describe a re-dating of a modern human jaw discovered in 2001 in the Misliya cave at Mount Carmel in Israel. After a very careful re-evaluation, using different high-tech methods, the large team of researchers confirmed the attribution to modern Homo sapiens. However, they arrived at a surprising and solid new dating for this specimen at between 177,000 and 194,000 years before the present, which makes it the oldest evidence for modern humans outside of Africa. The problem is, that the standard narrative maintained that modern human originated about 200,000 years ago in Northeast Africa and only much later (about between 60,000 and 70,000 years ago) left Africa to spread around the globe (Callaway 2018b). Single discoveries of modern humans in the Levant before this time, like the 80,000-120,000-year-old remains from Skhul cave and Qafzeh in Israel, were considered to be failed early attempts to leave Africa that left no later descendants. The new results and other recent findings show that such ad hoc hypotheses to explain away conflicting evidence are no longer tenable. That is why the Times of Israel cites Professor Israel Hershkovitz as saying, “This has changed the whole concept of modern human evolution…The entire narrative of the evolution of Homo sapiens must be pushed back by at least 100,000-200,000 years.” The author of the article comments, “With this Misliya cave jawbone, however, the history of human evolution is being rewritten” (Borschel-Dan 2018, emphasis added).

Where have we heard that before? Dienekes Pontikos (2018), who hosts one of the most popular paleoanthropological blogs, notes the new results with the headline, “Out of Africa: a theory in crisis,” and remarks that it “should cause a rethink of the currently held Out-of-Africa orthodoxy.” Hear, hear!

An Initial Couple? 

Unfortunately Smithsonian Magazine’s list of top discoveries of the 2010s fails to recognize a much bigger story that answered a question of interest to a broad segment of the population: Did humans arise from an initial couple? Between 2016 and 2019, qualified scientists for the first time developed a model to rigorously test the long-assumed evolutionary viewpoint that modern humans originally descended from a large population composed of thousands of individuals. This study resulted in three publications which ultimately found that it is possible that modern humans originated not from a large population but from an initial pair that lived as recently as 500,000 years ago (or potentially even more recently, if fewer assumptions are made).

According to the final paper of this study, published in 2019 by biologist Ann Gauger and mathematician Ola Hössjer in the journal BIO-Complexity, within human origins research “certain common assumptions used for convenience have been misinterpreted as if they were data-driven conclusions, without testing the single-couple origin hypothesis scientifically.” Their model tries to test the question of whether humanity could have originated from an initial pair, and it seeks to do this by minimizing assumptions:

The SOC [Single-Origin Couple] model we consider differs from the prevailing interpretation of human ancestry in only two assumptions: (i) That we evolved continuously from non-humans in a large population. (ii) That genetic diversity is always due to germline mutations.

Rather than forcing models to assume that genetic diversity must only derive from germline mutations and that population sizes must always be very large, their paper allows that an initial couple of human beings could have been “instantiated” with “primordial diversity.” 

This study is a great example of how we must allow a model to be tested by fairly implementing its boundary conditions. It shows precisely why it’s wrong to use circular reasoning to first assume that unguided evolutionary boundary conditions are the only options, and then conclude that other models must therefore be wrong. After implementing their boundary conditions and applying the mathematics of population genetics, the authors conclude that, “as far as we know scientifically from the genetic data, the human species could have come from as a single couple, so that all humans alive today could have descended uniquely from that first pair.” They found that this initial couple could have lived as recently as 500,000 years ago. For more details on this research, see Uniqueoriginresearch.org.

Of course the Smithsonian Institution is entirely wedded to an effectively unguided evolutionary view of human origins, as their Hall of Human Origins in Washington, D.C., and their traveling human origins exhibit have made clear in recent years. Thus it is unsurprising that they ignore studies that challenged prevailing evolutionary wisdom, and positively spun discoveries in the 2010s that actually weakened the evidence for human evolution. If anything, these discoveries show that the 2010s were not a great decade after all for the paleoanthropological evidence for human evolution.

Perhaps in ten years we’ll be having this conversation again — and perhaps at that time the Smithsonian Institution will give us all a more objective analysis of the evidence. Who knows what the next decade will bring?

Photo: Foot of Homo naledi, by Lee Roger Berger research team [CC BY 4.0], via Wikimedia Commons.