Recently, Evolution News and Views published an article titled, The Human-Ape Missing Link — Still Missing (July 18, 2017), which attempts to cast doubt on human evolution by quoting from a recent BBC article which highlighted the massive uncertainties that still remain over the identity, appearance and date of the last common ancestor (LCA) of human beings and chimpanzees, and which even questions whether the chimpanzee is our closest relative, after all. The Evolution News and Views (ENV) article also revives the myth of an unbridgeable gap between Australopithecus and Homo.
Here’s my two-sentence rebuttal: uncertainty as to who the last common ancestor of humans and chimps was, what it looked like, and when it lived, in no way diminishes scientists’ certainty that it existed. And while the fossil record of human ancestors is very meager and patchy until about 4.4 million years ago, from that time onward, we have a veritable hodgepodge of hominins – and no unbridgeable gaps.
Well, that was quick, wasn’t it? Now for a more detailed rebuttal.
What’s the best evidence for human evolution?
For those wanting a quick overview of the evidence for human evolution, I would recommend Dennis Venema’s four-part series, titled, Intelligent Design and common ancestry, as well as his five-part series, Vitellogenin and Common Ancestry.
Dennis Venema’s 22-part series, titled, Adam, Eve, and Human Population Genetics, goes on to establish that the ancestral human population has never numbered less than 10,000 during the past one million years. In short: the notion that humanity originally descended from a single couple has been scientifically discredited. After reading Venema’s posts, I have been forced to revise my own views on Adam and Eve. (More about that in a future post.)
Readers who may be inclined to query the alleged 98% genetic similarity between humans and chimps might want to have a look at my Uncommon Descent articles, Human and chimp DNA: They really are about 98% similar and Double debunking: Glenn Williamson on human-chimp DNA similarity and genes unique to human beings. The latter article also deals with the genes which are alleged to be totally unique to human beings, and shows that they are nothing of the sort.
What the BBC article actually said
The BBC article cited in the anonymous Evolution News and Views post on human evolution was refreshingly frank about what we do and don’t know about the last common ancestor of humans and chimpanzees. Here’s how the ENV post summarized its findings:
Here is a long, substantive, and interesting article from the BBC — “We still have not found the missing link between us and apes.” It is interesting for two reasons.
1. It admits that we haven’t found anything that resembles the last common ancestor (LCA) between humans and apes, what author Colin Barras calls the “missing link.”
2. It admits that it’s hard to even agree on what the LCA might have looked like.
Nowhere, however, did the article contest the evidence for human evolution. What it said was that last common ancestor may have looked a lot less chimp-like than most scientists had previously believed, and that we still haven’t found this creature yet.
My major quarrel with the BBC article is that it did not mention any hominin fossils (in the line leading to humans) older than that of the 4.4-million-year-old Ardipithecus ramidus. Older probable hominin fossils such as Ardipithecus kadabba, Orrorin tugenensis, Sahelanthropus tchadensis and Graecopithecus freybergi, which go back as early as 7.2 million years ago, are completely ignored. This is a significant omission, as the human-chimp split is currently estimated to have taken place between 7 and 13 million years ago. If the oldest hominin fossil is 7.2 million years ago, then we are not so far from the last common ancestor, after all.
The allegedly unbridgeable gap between Australopithecus and Homo
The Evolution News and Views article also claims that “we don’t even have transitional forms between Australopithecus and Homo.” This, I have to say, is simply not true.
The article they link to, to support this claim, is an earlier post on Evolution News and Views (“A Big Bang Theory of Homo,” August 13, 2012). Unfortunately, the references cited nearly all date from 1990 to 2007. In recent years, there has been a dramatic shift in scientists’ views.
Until a few years ago, many anthropologists believed that there were stark anatomical differences between Australopithecus and Homo ergaster (pictured above), whose Asian counterpart was Homo erectus. Many of these anthropologists also believed that Homo habilis should be classified as a species of Australopithecus. Oft-cited in this regard is a 2000 paper by J. Hawks, K. Hunley, S.H. Lee, and M. Wolpoff, titled, Population bottlenecks and Pleistocene human evolution (Molecular Biology and Evolution 17(1):2–22), in which the authors write: “We, like many others, interpret the anatomical evidence to show that early H. sapiens was significantly and dramatically different from earlier and penecontemporary australopithecines in virtually every element of its skeleton (fig. 1) and every remnant of its behavior…” In support of their claim, the authors cited the work of Bernard A. Wood and Mark Collard, who put forward powerful arguments for this view in 1999, in their paper, The human genus (Science Vol. 284 no. 5411 pp. 65-71). However, I should mention that while Wood and Collard found major anatomical differences between Homo habilis in six broad categories of traits – body size, body shape, locomotion, jaws & teeth, development, and brain size – three of those traits could not be assessed for another species of early Homo, Homo rudolfensis. Wood and Collard defended their view that Homo erectus represented a clean break from his hominid predecessors once again in their 2001 paper, The Meaning of Homo (Ludus vitalis, vol. IX, no. 15, 2001, pp. 63-74) and more recently, in their 2007 paper, Defining the genus Homo (in Henke, W. and Rothe, H. and Tattersall, I., (eds.) Handbook of Paleoanthropology, Springer Berlin Heidelberg: Berlin, pp. 1575-1610).
However, the scenario proposed by Wood and Collard and is now out of date. Recent papers published in 2012 – see Early Homo: Who, When, and Where (by Susan C. Antón, in Current Anthropology, Vol. 53, No. S6, “Human Biology and the Origins of Homo,” December 2012, pp. S278-S298), Origins and Evolution of Genus Homo: New Perspectives (by Susan C. Antón and J. Josh Snodgrass, in Current Anthropology, Vol. 53, No. S6, “Human Biology and the Origins of Homo,” December 2012, pp. S479-S496) and Human Biology and the Origins of Homo: An Introduction to Supplement 6 (by Leslie C. Aiello and Susan C. Antón, in Current Anthropology, Vol. 53, No. S6, “Human Biology and the Origins of Homo,” December 2012, pp. S269-S277) show that the transition from Homo habilis to early Homo ergaster / erectus was not much larger than that between Australopithecus and Homo habilis. A detailed anatomical comparison indicates that the transition from Australopithecus to early Homo, who appeared about 2.3 or 2.4 million years ago, and from early Homo to Homo ergaster / erectus, is much smoother and more gradual than what anthropologists believed it to be, ten years ago. The above-cited 2012 article by Susan C. Antón and J. Josh Snodgrass, titled, Origins and Evolution of Genus Homo: New Perspectives, conveys the tenor of the new view among anthropologists (emphases mine – VJT):
Recent fossil and archaeological finds have complicated our interpretation of the origin and early evolution of genus Homo. It now appears overly simplistic to view the origin of Homo erectus as a punctuated event characterized by a radical shift in biology and behavior (Aiello and Antón 2012; Antón 2012; Holliday 2012; Pontzer 2012; Schwartz 2012; Ungar 2012). Several of the key morphological, behavioral, and life history characteristics thought to first emerge with H. erectus (e.g., narrow bi-iliac breadth, relatively long legs, and a more “modern” pattern of growth) seem instead to have arisen at different times and in different species…
Over the past several decades, a consensus had emerged that the shift to humanlike patterns of body size and shape — and at least some of the behavioral parts of the “human package” — occurred with the origin of Homo erectus (e.g., Antón 2003; Shipman and Walker 1989). This was seen by many researchers as a radical transformation reflecting a sharp and fundamental shift in niche occupation, and it emphasized a distinct division between H. erectus on the one hand and non-erectus early Homo and Australopithecus on the other. Earliest Homo and Australopithecus were reconstructed as essentially bipedal apes, whereas H. erectus had many of the anatomical and life history hallmarks seen in modern humans. To some, the gap between these groups suggested that earlier species such as Homo habilis should be excluded from Homo (Collard and Wood 2007; Wood and Collard 1999).
Recent fossil discoveries paint a picture that is substantially more complicated. These discoveries include new fossils of H. erectus that reveal great variation in the species, including small-bodied members from both Africa and Georgia (Gabunia et al. 2000; Potts et al. 2004; Simpson et al. 2008; Spoor et al. 2007), and suggest a previous overreliance on the Nariokotome skeleton (KNM-WT-15000) in reconstructions of H. erectus. Additionally, reassessments of the Nariokotome material have concluded that he would have been considerably shorter than previous estimates (∼163 cm [5 feet 4 inches], not 185 cm [6 feet 1 inch]; Graves et al. 2010), younger at death (∼8 years old, not 11–13 years old; Dean and Smith 2009), and with a life history pattern distinct from modern humans (Dean and Smith 2009; Dean et al. 2001; Thompson and Nelson 2011), although we note that there is substantial variation in the modern human pattern of development (Šešelj 2011). Further, the recent discovery of a nearly complete adult female H. erectus pelvis from Gona, Ethiopia, which is broad and has a relatively large birth canal, raises questions about the narrow-hipped, Nariokotome-based pelvic reconstruction and whether H. erectus infants were secondarily altricial (Graves et al. 2010; Simpson et al. 2008).
I should add that the average brain size of Homo ergaster / erectus specimens in Africa, dating from 1.8 to 1.5 million years ago, is a mere 863 cubic centimeters, while that of Georgian specimens of Homo ergaster / erectus dating from 1.8 to 1.7 million years ago is even lower, at 686 cubic centimeters (see Susan C. Antón and J. Josh Snodgrass, from Origins and Evolution of Genus Homo: New Perspectives, in Current Anthropology, Vol. 53, No. S6, “Human Biology and the Origins of Homo,” December 2012, pp. S479-S496). By comparison, the brain size of early Homo specimens (excluding 1470 man) is 629 cubic centimeters. [The brain size of 1470 man, or Homo rudolfensis, is variously estimated at anywhere between 526 and 752 cubic centimeters.] Quite clearly, there is no evidence for a sudden jump in brain size from Australopithecus afarensis (whose average brain size was 478 cubic centimeters) to Homo ergaster / erectus. The brain size of early Homo (who lived around 2.3 million years ago) is intermediate between the two.
Is the chimp our nearest relative, or is it the orangutan?
The Evolution News and Views post on the missing link also questions the molecular data linking human beings to chimpanzees, citing the work of Jeffrey Schwartz (who is referred to in the BBC article which it quotes from). Schwartz contends that our nearest relatives are orangutans, not chimpanzees. In a 2009 paper, John Grehan and Jeffrey Schwartz argued that orangutans were morphologically closer humans than chimps were. (See here for a summary of their arguments.) However, another more recent study using a larger dataset found that chimpanzees are morphologically closer to humans than orangutans are (see also here).
The BBC article also pointed out that “few researchers agree with Schwartz.”
In short: the claim that humans are anatomically closer to orangutans appears doubtful. In view of the unequivocal molecular evidence linking humans to chimps, I think it would be prudent to go with the mainstream view that chimps are indeed our closest relatives.
When did humans split off from the line leading to chimpanzees?
Family tree showing the extant hominoids: humans (genus Homo), chimpanzees and bonobos (genus Pan), gorillas (genus Gorilla), orangutans (genus Pongo), and gibbons (four genera of the family Hylobatidae: Hylobates, Hoolock, Nomascus, and Symphalangus). All except gibbons are hominids. Image courtesy of Wikipedia and Fred the Oyster.
Ever since the late 1960s, molecular biologists have argued that humans and chimpanzees last shared a common ancestor around five or six million years ago. Gorillas were subsequently estimated to have diverged from the human-chimp line about seven million years ago. Recently, however, these specialists have revised their dating, and some researchers in the field are now doubling their original estimate of the date of the human-chimp split.
In 2012, a report in Nature by Aylwyn Scally et al. estimated that humans and gorillas last shared a common ancestor 10 million years ago, while humans and chimps diverged around 6 million years ago.
In 2014, however, a new study by Gil McVean et al. (vol. 483, 169–175 (08 March 2012), doi:10.1038/nature10842) suggested a much older date for the human-chimp split. A Live Science report by Charles Quoi (Human and Chimp Genes May Have Split 13 Million Years Ago, June 12, 2014) summarizes the results of the study (emphases mine – VJT):
Past estimates of when the ancestors of humans diverged from chimps suggested the most recent common ancestor of both species lived about 6 million years ago. However, in the past decade or so, genetic analyses revealed the human mutation rate is actually half as fast as was previously thought, suggesting the most recent common ancestor of humans and chimps actually lived at least 12 million years ago.
Now a new study of chimp mutation rates appears to confirm that the most recent common ancestor of humans and chimps lived about 13 million years ago.
“Our results add substance to the idea that the human-chimpanzee split was considerably older than has been recently thought,” said study co-author Gil McVean, a geneticist at the Wellcome Trust Centre for Human Genetics in Oxford, England.
Quoi qualifies his remarks by acknowledging that the new evidence is compatible with the human-chimp split having taken place as recently as 7 million years ago:
Paleoanthropologist John Hawks at the University of Wisconsin-Madison, who did not participate in this study, noted that 13 million years is only the average time for when the genes of the ancestors of humans and chimps diverged; it’s not necessarily when the ancestors of humans and chimps split into different species.
“A species divergence of 7 million to 10 million years would be just fine with a genetic divergence averaging 13 million years if the common ancestor population was very large in numbers, or the common ancestor population was spread into different subpopulations with reduced mixing between them,” Hawks said. [8 Humanlike Behaviors of Primates]
McVean agreed with Hawks’ analysis. If the size of the ancestral population of both humans and chimps was very large, then their common gene pool may have begun diversifying long before the ancestors of humans and chimps split into different species, he said.
In the last year, more recent research has lent further support to the view that humans and chimps may have split up to 12 million years ago. I’ll mention just two articles:
(1) Variation in the molecular clock of primates (PNAS, September 20, 2016; 113(38): 10607–10612) by Priya Moorjani, Carlos Eduardo, G. Amorim, Peter F. Arndt and Molly Przeworskia:
Events in primate evolution are often dated by assuming a constant rate of substitution per unit time, but the validity of this assumption remains unclear. Among mammals, it is well known that there exists substantial variation in yearly substitution rates. Such variation is to be expected from differences in life history traits, suggesting it should also be found among primates. Motivated by these considerations, we analyze whole genomes from 10 primate species, including Old World Monkeys (OWMs), New World Monkeys (NWMs), and apes, focusing on putatively neutral autosomal sites and controlling for possible effects of biased gene conversion and methylation at CpG sites. We find that substitution rates are up to 64% higher in lineages leading from the hominoid–NWM ancestor to NWMs than to apes. Within apes, rates are ∼2% higher in chimpanzees and ∼7% higher in the gorilla than in humans. Substitution types subject to biased gene conversion show no more variation among species than those not subject to it. Not all mutation types behave similarly, however; in particular, transitions at CpG sites exhibit a more clocklike behavior than do other types, presumably because of their nonreplicative origin. Thus, not only the total rate, but also the mutational spectrum, varies among primates. This finding suggests that events in primate evolution are most reliably dated using CpG transitions. Taking this approach, we estimate the human and chimpanzee divergence time is 12.1 million years, and the human and gorilla divergence time is 15.1 million years.
(2) New geological and palaeontological age constraint for the gorilla–human lineage split (Nature, vol. 530, pp. 215–218 (11 February 2016) doi:10.1038/nature16510) by Shigehiro Katoh et al., argues strongly that humans last shared a common ancestor with the gorilla at least 8 million years ago, and possibly 10 million years ago:
The palaeobiological record of 12 million to 7 million years ago (Ma) is crucial to the elucidation of African ape and human origins, but few fossil assemblages of this period have been reported from sub-Saharan Africa. Since the 1970s, the Chorora Formation, Ethiopia, has been widely considered to contain ~10.5 million year (Myr) old mammalian fossils. More recently, Chororapithecus abyssinicus, a probable primitive member of the gorilla clade, was discovered from the formation. Here we report new field observations and geochemical, magnetostratigraphic and radioisotopic results that securely place the Chorora Formation sediments to between ~9 and ~7 Ma. The C. abyssinicus fossils are ~8.0 Myr old, forming a revised age constraint of the human–gorilla split. Other Chorora fossils range in age from ~8.5 to 7 Ma and comprise the first sub-Saharan mammalian assemblage that spans this period. These fossils suggest indigenous African evolution of multiple mammalian lineages/groups between 10 and 7 Ma, including a possible ancestral-descendent relationship between the ~9.8 Myr old Nakalipithecus nakayamai and C. abyssinicus…
The authors are uncertain as to whether Nakalipithecus nakayamai lived just before or just after the split between the gorilla lineage and the line leading to humans and chimps.
So at the moment, any date between 7 and 12 million years appears possible, for the human-chimp split. For my part, I’ll go with a date of 8 or maybe 9 million years for the human-chimp split, and 10 to 12 million years for the split between gorillas and the human-chimp line. I should mention that a date of 8 million years for the human-chimp split would push back the date of the split between orangutans and the other great apes to as early as 20 million years ago, which is about as early as the fossil evidence will allow.
Who was the last common ancestor, anyway?
Curiously, the BBC article cited by ENV says nothing about Sahelanthropus tchadensis (pictured above and at the top of this post), a hominin who lived 7 million years ago in Chad. Its discoverers claimed that it was a very ancient human ancestor but not a chimpanzee ancestor; however, some experts now believe it may have been the common ancestor of humans and chimps – or a very near relative.
Another possible candidate for the last common ancestor of humans and chimps is Graecopithecus freybergi, who lived on the savanna in Greece about 7.2 million years ago. However, a detailed examination of the morphology of the molar teeth associated with two fossils of Graecopithecus freybergi published in 2017 suggests that it may be a hominin, although other experts are skeptical.
Finally, Oreopithecus bambolii, an extinct hominoid primate discovered in the 1950s that lived in Italy from 9 to 7 million years ago, has been proposed by some as a human ancestor, although most scientists view it as an ape who was not part of the human lineage, and regard its anatomical resemblances to humans (notably in its hands) as an example of convergent evolution. To this day, the creature’s taxonomic status remains hotly disputed.
To sum up: the last common ancestor of humans and chimps may have already been found. But if the human-chimp split occurred 8 or 9 million years ago, the existence of possible hominin fossils from 7.2 million years ago is definitely a hopeful sign.
Despite the many uncertainties regarding the timing of the human-chimp split and the identity and appearance of the last common ancestor, scientists are justifiably confident that the chimpanzee is our closest relative. There is no good evidence for a “Big Bang” in human evolution, as far as the evolution of the human body is concerned; nor was there a time when our brain size suddenly increased. To sum up: the recent ENV article, which uses the absence of the missing link in order to cast doubt on human evolution, is profoundly misguided.