Aaron Filler, MD, PhD, FRCS is the author of Do You Really Need Back Surgery?, and an evolutionary biologist who studied under Stephen Gould, Ernst Mayr, David Pilbeam, Russell Tuttle, and Irven DeVore. Filler is now a medical director at the Institute for Spinal Disorders, Cedars Sinai Medical Center, Los Angeles. In the article below he looks at what makes humans- “human.”
OED: “Human”– Adjective – Of, belonging to, or characteristic of mankind, distinguished from animals by superior mental development, power of articulate speech, and upright posture.
It’s easy enough for anyone to tell the difference between a human and an ape today, but how easy would it have been six million years ago, around the time of the split between the lineage of modern humans and the lineage of the chimpanzees. If you were to see the common ancestor would you think “human” or would you think “ape?”
Over the past 50 years there has been an understanding in the scientific community that the common ancestor would look somewhat like a knuckle-walking chimpanzee and that the various descendant lineages on the human side would more or less gradually begin to stand upright and walk bipedally on two legs. The common ancestor was a lowly quadrupedal ape, but our direct predecessors then gloriously stood upright on their two legs and eventually strode their way across the border between animality and humanity.
For many years, there was no solid fossil evidence to support this understanding. Now we have dozens of relevant fossils but they all seem to show that this scenario is wrong. In fact, the common ancestor may well have looked much more like a human than like an ape.
This sharpens the question, – is there a quintessential aspect of an animal that makes it a human? Can this be identified on a discreet biological basis so that the relevant critical gene changes can be spotted by genome researchers?
This year, it has become apparent that a new morphogenetic basis for defining humanity may have become known (Homeotic Evolution in the Mammalia… a Morphogenetic Basis for Human Origins – PloS ONE 2007). It is a dramatic restructuring of the lumbar vertebra that inverts the mechanics of the spine and undoubtedly underlies our upright bipedal posture. The problem is that the new definition reveals that some of the apes appear to have descended from upright bipedal (human) ancestors.
To some extent, we can dodge the issue of what to call these bipedal ancestor species by the use of terms like: “proto-human,” “early human,” “ancestral human,” “hominine,” or “hominid.” This doesn’t really help very much since we still want to know if the common ancestor six million years ago looked more like an ape or more like a “proto-human.” Terms like hominine and hominid have increasingly been rendered unusable for this purpose because they are based on formal systematic family and sub-family names and their usage is in constant flux – they have both been used recently to describe groups that include apes. Therefore, whether “proto-“, “ancestral” or “early” many of the upright bipedal ancestor species get to be called some kind of “human.”
When we learn about the stress on the lumbar curvature in pregnant australopithecine females who must walk bipedally with their growing abdominal weight and girth we understand that these creatures share a quintessentially human problem with our modern species (see Evolutionary Origins of Back Pain, and also Lumbar Lordosis in Pregnancy). Understanding a human as a type of animal with our body plan (bauplan) is what underlies this perception.
If you could see Lucy (Australopithecus afarensis) walking along 3.2 million years ago with her chimp-like head, but upright bipedal body – many of us would think “human!” But when did the human body plan first appear and is the body plan alone sufficient to win the sacred moniker of “human?”
The OED definition requires articulate speech and superior mental development to identify a human. It is not too surprising that the spine could hold the keys to upright posture, but what about the rest of what makes us human? It is now completely clear that species such as Australopithecus afarensis (Lucy) had brains and skulls little different from some modern apes, yet there is universal agreement that australopithecines are not apes.
Video from a recent study showing that chimpanzees consistently outperform human college students on a complex computer based eidetic memory task shows that the common ancestor’s intellect was at least respectable. However, despite being “human,” australopithecines almost certainly did not have articulate speech or “superior” mental development.
It seems that we are unavoidably forced to abandon articulate language and superior intellect as requirements for our critical definition of a human. If Homo erectus or Homo habilis couldn’t engage in ‘articulate speech’ are they species of apes? What about Australopithecus africanus? The search for the essence of our humanity now focuses upon the anatomy and genetics of the spine.
We are all interested in our spine when we suffer from back pain – but it is increasingly also clear that vertebrae are more important than anything else in defining humanity.
It is not surprising that human lumbar vertebrae are radically different in architecture and function than the lumbar vertebrae of a monkey. What is surprising is that all of the major transformations that underlie the uniqueness of the human lumbar vertebrae first appear in the fossil record 21.6 million years ago in a fascinating hominoid species Morotopithecus bishopi.
Recently, I have pointed out that a critical single change in one of our hominoid morphogenes could have generated our unique human body plan. This body plan appears to be almost entirely a consequence of this single morphogenetic evolutionary event. It generated the unusual vertebral structure first seen in Morotopithecus that is still preserved in its original primitive form in modern humans. The article is published in PLoS ONE (the brash new online competitor to Science and Nature).
As I outline in Do You Really Need Back Surgery – the spine is a structure made up of repeating segments and those segments have an incredibly ancient origin in the history of animal biology. Modern genetics has revealed that vertebrae are distant echoes of the grand evolutionary events of the Cambrian Explosion that took place some 522 million years ago. A major new sub-field in genetics analyzes the morphogenetic homeotic genes that organize the shape and growth of body segments and this sub-field has grown explosively over the past twenty years. It is moving relentlessly towards the very center of evolution biology.
The importance of segments in biological structure first entered the scientific mainstream in the biological writings of the poet Johann Wolfgang von Goethe in the 1790s and reached a first pinnacle of attention at the great academy debate between Etienne Geoffroy Saint-Hilaire and George Cuvier in 1830 (see my recent books “The Upright Ape” – New Page Books 2007 with a foreword by David Pilbeam, Dean of Harvard College, and Axial Character Seriation in Mammals, and also see Toby Appel’s “The Cuvier-Geoffroy Debate” – OUP 1987). “Homeotics” is a term that describes the study of how the segments of the animal body are differentiated from one another by their genes.
Only a tiny fraction of our DNA is concerned with organizing the physical shape of our bodies, but these “morphogenes” are very important in evolutionary biology. For one thing, their effects are virtually all we can see when we look at a fossil. Two other aspects of morphogenes are quite stunning relative to many other genes – firstly, they are spectacularly conservative across time – the gene complexes that determine the organization of insect segments have an astonishing level of similarity with the ones that control the organization of mammalian vertebral segments. Secondly, a very small change in a single morphogene can have astonishing effects on the appearance of the resulting adult animal. A single DNA base-pair changes and a fruit fly has fully formed legs on its face, etc.
This second issue pertains to the fact that morphogenes control the organization of complex body modules. The result is that a small single genetic change can generate an entirely new type of animal in a single generation. For this reason the role of sudden change in biological evolution and the evolution of morphogenes have become bedfellows. In our new definition of humans, I propose that our body plan arose through a small change in a Pax morphogene and that the upright bipedal body form arose suddenly in a single generation rather than gradually under pressure from natural selection across millions of years.
Within biology there has been an epic intellectual debate that has persisted unabated across the 148 years of our post-Darwinian era. Is all of evolution painted out in gradual shifts of small details or are there sometimes sudden transforming events in which new kinds of animals arise? This debate carried Stephen Jay Gould onto the front cover of Newsweek in 1982 and lay behind decades of conflict between Gould and Ernst Mayr. Most recently, the rising primacy of morphogenetics has made Sean Carroll one of the leading voices in evolutionary theory (The Making of the Fittest, From DNA to Diversity) putting him at the opposite end of the biological spectrum from devout traditional gradualists such as Richard Dawkins (The Selfish Gene ).
Is Darwinian gradualism best suited for fine tuning little details such as the shape of the beaks of finches, or is it so powerful that all change in biology are always an accumulation of a large number of small gradual alterations? Goethe was the first to point out striking evidence that animals and plants were assembled from repeating elements. Geoffroy suggested a fundamental similarity between insect segments and vertebrae – an outrageous idea that more or less ruined him academically, but an idea we now know to be not only true, but extremely fundamental to understanding change in animal form. Today we call these biological modules and their proper study is through Modularity Theory (Modularity in Development and Evolution – Univ. of Chicago Press 2004).
In The Upright Ape I point out two convincing examples of sudden “non-Darwinian” evolution. The first of these concerns the origin of a group we now call the Bilaterians because of their bilateral symmetry – this includes the insects and the vertebrates and a wide variety of other groups and phyla that have right-left paired limbs and sense organs. Prior to the Bilaterian origin there were animals with no particular symmetry (unicellular), branched animals (sponges) and radially symmetric animals (jellyfish). No one really argues that the original “Ur-Bilaterian” arose gradually, across millions of years as a unilaterally structured animal lineage gradually grew a right side attached to its already fully formed left side. No – as I show in my PLoS ONE paper, our morphogenes are subject to duplication and mirroring. The first bilaterally symmetric animal in the Bilaterian lineage was one individual born with a right-left body duplication. Suddenly, without any roll for variation, natural selection, shifting gene frequencies etc – we had the first Bilaterian. The ancestor of all the bilaterally symmetric phyla including insects and us. This is the proto-typical non-Darwinian sudden evolutionary event. Darwinian evolution then engages to optimize, fine tune and diversify the results of this major innovation.
The second major event I point to is the origin of vertebrates (technically – deuterostomes). In most Bilaterians, the nerve cord is along the front surface of the animal and the digestive tract is along the back. Geoffroy argued that vertebrates emerged from invertebrates by being flipped 180 degrees. We now know that the ‘dorso-ventral read out gradient’ in the embryo was indeed inverted at the time of vertebrate origins. It was not a matter of gradually shifting a few degrees every million years until 180 degrees were reached. No, a single offspring was born with a small morphogenetic gene change that resulted in an upside down animal – natural selection played a role in that this “hopeful monster” survived and founded a lineage.
In this way, not only Bilaterians, but then Vertebrates came to be through non-Darwinian events. Like Newtonian physics and Einsteinian physics – these two modes of biological change appear to co-exist. Most of the time we see only Darwinian evolution, but this is just an approximation of the fine tuning effects of natural selection upon the body plan generating capabilities of the morphogenes.
In human origins, I have pointed out that the fundamental mechanical architecture of the lumbar spine in most primates underwent a 180 degree flip in Morotopithecus bishopi in the Early Miocene of 21.6 million years ago. Technically speaking this “septo-neural transposition” (SNT) is the fundamental synapomorphy (shared derived feature) that defines the hominiform (human shaped) lineage.
This appears to be a non-Darwinian event leading to the first ape child with a human body plan – standing fully upright in a family of “proconsulid” apes that were fully quadrupedal. In my recent video “Hominiform Progression” I show that in one of the earliest groups of hominoids to branch off after Morotopithecus, the infants do the same thing that human infants do – they teach themselves spontaneously to walk bipedally as their primary form of locomotion. Since all of the hominiformid hominoids that descend from the lineage of Morotopithecus share the same vertebral transformation and since the two widely separated groups have infants that spontaneously walk bipedally, I have pointed out that the Morotopithecus babies of 21 million years ago probably did the same thing – taught themselves to walk bipedally.
I have categorized hominiform species as being “eubipedal hominiforms” if their principal means of locomotion on the ground or large branches is bipedalism (humans and hylobatids) and as being “metabipedal hominiforms” if they only occasionally use bipedalism but have primarily replaced this with knuckle walking or fist walking in a diagonal or horizontal posture when traveling on the ground (chimps, gorillas and orangutans) (see the Hominiform Progression video).
Among the eubipedal hominiforms, the hylobatids have diverged to take on brachiation (arm swinging) as their primary means of travel – even though they are better bipeds than humans when walking on vines and branches (see figure above). However, any eubipedal hominiform that does most of its locomotion via bipedalism should probably be called a human. This may well include Morotopithecus (apparently not all that well adapted for arm swinging) as well as other hominoids from before the chimp-human split such as Oreopithecus, Pierolapithecus, Orrorin and Sahelanthropus.
On this basis, I suggest that there has been a continuous lineage of species that are primarily bipedal with the human body plan from 21 million years ago to today. The various lineages leading to chimps, gorillas, and orangutans will have branched off and evolved their unique types of diagonal and horizontal body posture millions of years after the emergence of their human ancestors. The relatively great success of the “human” hominiforms is suggested by the fact that all of the fossils we find appear to be from upright bipeds.
Among the most interesting of the apparently upright bipedal fossil hominoids is Sahelanthropus from seven million years ago (The First Human). It appears to meet the criteria for being called a human just as Australopithecus afarensis (Lucy) does. However it seems to have lived before the chimp-human split at 6 million years ago as suggested by molecular projections. On this basis I have argued that there appears to be a human ancestor for the chimpanzees (anthopology.net).
So far, all of the fossils of hominiforms predating the chimp-human split that have been found are from upright, hindlimb supported species – this includes Morotopithecus, Oreopithecus and Pierolapithecus based on vertebrae, Orrorin based on the femur and Sahelanthropus based on the skull. No fossil evidence of knuckle-walking ancestors has been discovered. Many of these “pre-split” hominiform species appear to meet a new definition of a human – a hominifom hominoid having the SNT spinal trait that retains bipedalism as its primary mode of locomotion. Therefore all of the lineages of apes alive today may have descended from human ancestors.
Aaron Filler, MD, PhD, FRCS is an evolutionary biologist who studied under Stephen Gould, Ernst Mayr, David Pilbeam, Russell Tuttle, and Irven DeVore and who is now a medical director at the Institute for Spinal Disorders, Cedars Sinai Medical Center, Los Angeles