Homeotics, Cladistics and the Triple Emergence of Closed Hand Gaits in Descendants of an Upright Ancestor of the Apes
Guest Post by Aaron Filler, MD, PhD, FRCS
If an upright bipedal ancestor evolved first, why would three of its descendant lineages abandon orthograde posture and restore quadrupedal gaits to their repertoire?
Firstly, we don't have to have an answer as to why it happened if that is indeed what happened. If it occurred, then it occurred whether or not we have proposed and accepted a post-hoc adaptationist explanation for it.
This week in PNAS, a paper from Tracy Kivell and Daniel Schmitt, reports on differences between the functional anatomy of knuckle walking of gorillas as distinct from the basis in chimpanzees, strongly supporting the idea that these are two similar but independently evolved locomotor complexes. They go on to argue that humans therefore did not evolve from a knuckle walking ancestor.
These authors have focused on significant differences in the carpal bones that underlie the two forms of knuckle walking. There is a long and heated history of publications in this area (see the Upright Ape for a more detailed discussion of the knuckle walking controversy). Kivell and Schmitt add significant new information about the mechanics and anatomy of knuckle walking as it relates to the very different carpal bone structure in the two lineages. This work does greatly strengthen the argument that there is no proven basis for making these two adaptations into a single shared change that should have been present in the group ancestral to hominins.
On an adaptive basis, one of the arguments most strongly advanced for the preference of closed hand gaits in various hominoids is that the tendons are shortened to result in a passive "hook" of the hand when the arm joints and wrist are extended. This is for the purpose of efficient suspension from branches during hand hanging and locomotion. However, in the light of the Humanian model I have advocated,, there would be a multi-million year history of locomotion without hand/arm use and a behavioral reliance on use of the hands for carrying crude tools and small food objects. Closed hand gaits allow hominoids to hold objects in their hands as they locomote - it is easy enough for an adaptationist to explain the adaptive benefit of being able to carry during locomotion when you are in competition with an upright bipedal hominoid. Classical Darwinists often miss this point because of the focus on competition with other species members, but Stephen Jay Gould has pointed out the seemingly obvious importance of competition above the systematic level of the species. This is all about what upright bipeds and knuckle-walkers do on the ground when they compete for resources - and fight with each other - in overlapping territories.
The homeotic mutationist position that I have advocated (The Upright Ape, Homeotic Evolution of the Mammals) tackles the question from the perspective of genetic drive and adaptationist fine tuning. The starting point is the demonstration that the Moroto vertebra demonstrates a truly remarkable transformation of the vertebrate body plan. In most mammals and indeed in most vertebrates - the main dorso-ventral body plane division is placed ventral to the neuraxis, but in a "hominiform" clade that includes hominins, some extant apes, Morotopithecus, Pierolapithecus, Oreopithecus, Orrorin and Sahelanthropus the body plane is shifted to a position dorsal to the neuraxis in the lumbar region.
A number of anatomical transformations in serial homology accompany this change and all of them appear at 21.6 million years ago in Morotopithecus. These include a "box-like" cross section of the lumbar transverse process as opposed to the more standard mammalian flat element and this appears to be evidence of bipedal rather than bimanual orthogrady - e.g. standing and walking rather than arm swinging. The anatomical basis of upright posture in modern humans is very clearly due to this transformation and it is fully present in Morotopithecus. Because of this, I have argued that this feature is the basis of a hominiform clade that separated from the proconsuliform hominoids very soon after the first emergence of hominoids in the early Miocene.
Sudden major transformations of this type are the province of mutationist rather than adaptationist evolutionary progression and are best suited to changes in the major morphogenetic genes such as the homeotic genes. The prime irrefutable example that challenges traditional evolutionists who refuse to understand the implications of modern morphogenetics is the origin of the vertebrates from the invertebrates. Invertebrates have their digestive tract dorsal to their neuraxis, while vertebrates have the neuraxis dorsal to their gut. Invertebrates develop the mouth from the blastopore, but vertebrates develop the anus from the blastopore. To get from the invertebrate body plan to the vertebrate body plan, you have to flip dorsal for ventral and anterior for posterior. An adaptationist argues that there were selective adaptive pressures that gradually turned the proto-vertebrates a few degrees at a time over millions of years. The morphogentic mutationist realizes that this is completely ridiculous and that - as the genes show - the readout gradients in the embryo got flipped.
Vertebrates with the dorsal neuraxis and "aboral" blastopore emerged in abrupt single generation transformations that led to a revolutionary new body plan. In Morotopithecus bishopi we see a similar morphogenetic revolution and the result is the proto-human that does not use its hand or arms to locomote. This most likely occurred in a single generation. A quadrupedal proconsulid-like ape mother and a bipedal orthograde child that was our clade founder once its descendants were isolated by a chromosomal speciation event.
I point out in the Upright Ape that every fossil hominoid species in the hominiform group for which we have post-cranial fossil evidence appears to be an upright adapted form and that there is no evidence yet of any knuckle walkers. The accepted position has been that humans evolved from knuckle walkers around 5-6 million years ago even though there has never been any evidence for this at all. The contrary position that I have advocated is supported by all the evidence from all of the five separate genera of hominiform hominoids known with post-cranial evidence from before the chimp-human split, distributed across time from the 21 to 5 mya, yet my position is the one that has been considered radical.
The general dictates of science are that more credence should be given to a theory with a variety of elements of supporting evidence and less credence to a theory that fits expectations but for which evidence has not yet been found. There are numerous objections to the Humanian Model - a blog I wrote on this subject on Anthropology.net drew nearly 20,000 views and attracted scores of comments making up a good mix of hostile and supportive reactions. However this suggestion has had a very significant impact (Science Daily, blog on afarensis, blog on scientific blogging, MSNBC) because of the strength of the evidence. It was also well received when presented at the 2008 meeting of the American Association of Physical Anthropology and was addressed specifically in presentations others - who reached conclusions consistent with the components of the Humanian Model. David Pilbeam - past Chairman of Biological Anthropology at Harvard - wrote the foreword for the Upright Ape.
Although I first laid out the case in 1986 in Axial Character Seriation in Mammals (republished in 2007), the publication of my 2007 books and papers coincided with the publication of work by Thorpe and Crompton arguing for a upright climbing antecedent for our terrestrial bipedal lineage. There have been three more new hominoid species reported since mid-2007,,, but none of these finds has included significant new post-cranial fossils. However, analysis of spinal evolution by Rosenman and Lovejoy, is consistent with the Humanian model, an analysis of hand bones from Hispanopithecus laeitanus has demonstrated no sign of knuckle walking, and now the new report from Kivell and Schmitt on knuckle walking adds yet more weight to the non-Troglodytian models (no knuckle walking for the human ancestor).
Various studies show the inefficiency of chimpanzee bipedal walking and argue that only by a series of modifications could we arrive at the efficiency of the bipedalism of extant humans. However, this assumes that the common ancestor of chimpanzees and humans walked bipedally the way a modern chimpanzee does when a more correct scenario is probably that the common ancestor walked more as we do - and that its ancestors already had a 15 million year career of minor adaptations improving the effectiveness of bipedal gait.
In the Upright Ape, as well as in the Homeotic Evolution of Mammals and a paper on later Emergence and Optimization of Upright Posture for bipedalism, I update the original (1986) hypothesis by pointing out that the best way to view the locomotor specializations of modern orangs, chimps and gorillas is as examples of "diagonograde" posture. The basic hominiform spine - due to a homeotic mutation - is defective in that it loses the basic mechanical structure that supports the horizontal or pronograde spine in most mammals. This is what occurred in Morotopithecus and has never been restored. In most hominiforms, we retain a very flexible lumbar spine with five or six lumbar vertebrae. However in the orang, the chimp and the gorilla, the lumbar spine is dramatically shortened - to as few as three lumbar vertebrae and independently evolved bony stops that limit hyperextension of the non-orthograde spine appear - each quite independent and different in those three lineages.
Russell Tuttle, showed that Orangutans do not knuckle walk - they fist walk. Owen J. Lewis showed that there was no clear shared anatomical basis for knuckle walking in the hominoid wrist (see Upright Ape for numerous references). Kivell and Schmitt have now showed that there are indeed two very distinct functional anatomical patterns for knuckle walking that distinguish chimps and gorillas.
So...if a series of hominiform hominoids share the long flexible lumbar spine that can't function usefully in pronograde or diagonograde postures, if all of these genera of hominiforms demonstrate anatomical features for support upon pelvis below rather than by suspension, and if the orangs, chimps and gorillas have independently evolved short stiff lumbar spines and closed fist diagonograde gaits, then what stands against a Humanian model of human evolution?, Certainly there is a mountain of writing, publications, textbooks, theory and belief against it - but there is no actual scientific evidence to support anything other than an upright ancestor for the extant apes and humans.
I have further tried to look closely at what we mean by the word human (OUP blog: Redefining the Word "Human" - Do Some Apes Have Human Ancestors?). As I detail in the Upright Ape, we have Sherwood Washburn to thank for our anthropological definition of the word human and it is basically a hominoid that is bipedal and upright. Clearly, when we look at that century in the early Pliocene that immediately followed the speciation event that sealed the chimp-human split we would now have "humans." These humans would have a brain size no different from a chimp, tool use no different from a chimp, and communication no different from a chimp. The assumption has been that the common ancestor of chimps and humans would look like a chimp - an obvious "ape" - however, what if the common ancestor looked pretty much like the early humans. What if it is the human lineage that retains the primitive traditional hominiform body plan that has been around since the time of Morotopithecus?
I think we all know what we mean by "early human" - they have our basic body form but have smaller ape-like brains, shorter legs, a bigger set of jaws and teeth, and a big toe that spreads out further from the other toes than ours does now. Since the Oxford English Dictionary is probably the best forum for this question, it is on the OUP blog that I placed my piece on redefining the word "human".
From the point of view of cladistics, we really can't use the term "apes" anyhow since this does not form a monophyletic group. From that perspective, we would have to include humans as apes. In fact when you look at the actual sequence of events among the hominiforms and extant apes, "humans" came first, but then the cladists would have us calling the extant apes "humans" and that is not what anyone means. Hominiforms is a good cladistic term for the humans and extant apes. The word human - including early humans and proto-humans - is a good term under Ernst Mayr's view of biological naming in systematics (see discussion in the Upright Ape). We can also say Hominiforms of human body plan (HHBP). All of these issues arise because systematic diversification (speciation) does not necessarily coincide with morphological diversification - some primitive body plans are more effective than the modifications that appear in some descendants.
This is the specific point that Ernst Mayr attacked most specifically in challenging the Hennigian paradigm. In cladistics, whenever a new species arises, we have two daughter species and we say that the parent species is now extinct. Mayr felt that the typical situation was that you had a large slowly evolving population that occasionally emits a speciated branch with a subset of the genetic variability of the parent species. He felt it was biological nonsense to say that a species was now extinct if the genetic makeup of the parent species and the major daughter species were identical. In our situation - we would have a large parent species of human form out of which a small group becomes isolated that leads to the chimpanzees. We agree that the post-split group that is bipedal is now "human" but the pre-split group (from the century before the speciation event) is effectively identical. That pre-split group should be called "human" as well and then we have a human group ancestral to an extant ape lineage - at least in that situation.
One thing that all the naming and systematics logic does not affect is the apparent fact that the immediate ancestors of the all the extant clades of apes & humans had the appearance of modern humans in their body plan.
So we have hominoids that share a modified Y-5 molar and which includes the pronconsuliforms and the hominiforms. The hominiforms are basically upright and of human body form - or humanoid. From the early Miocene to the present, there has always been a lineage of upright primarily bipedal hominiform hominoids whose body plan is of human aspect. In essence these are creatures that don't use their hands and arms for locomotion.
Across time, various groups branch off that are more suited to ecologies and locomotor arenas where humans don't compete as effectively - that is because it has always been difficult for nonhuman-like hominoids to compete against those with human form. The proconsuliforms long since went extinct and the others have not succeeded sufficiently to leave any post-cranial fossils. Meanwhile, the upright bipedal hominiforms with a body plan of human aspect have left an increasingly abundant fossil record that spans 21 million years. They were not stooping forward and they may have been seen occasionally in trees (see my video of Siamang bipedalism and human brachiation: Hominiform Progression on You Tube, in high res on Vuze or as an iTunes podcast). At any point during this time span, if you were to encounter a hominoid it would most likely have been an upright bipedal species that when seen from a distance looked...well... human.
Kivell, T., & Schmitt, D. (2009). Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0901280106
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