Humans walked the earth millions of years earlier than we believed and your teeth might be the scales of a fish

A new analysis continues to confirm Charles Darwin’s hypothesis that the first step towards humanity was walking upright, and his thoughts on the matter in Descent of Man still ring true over 150 years later, an incredible feat.  Meanwhile, other scientists are finally beginning to solve the riddle of where teeth came from.  It’s Darwin’s world, we’re just living in it.

For decades, the world has been captivated by Lucy, the fossilized remains of an early hominid known as Australopithecus afarensis.  Lucy is relatively unique among most fossils in general:  Some 40% of her remains were found in Ethiopia in 1974, making her far more complete a specimen than scientists normally have to work with, where in many cases they are reduced to drawing conclusions based merely on teeth or other bits of bone.  Lucy was special, and not simply because of the large quantity of her remains discovered.  In many ways, she represented the near mythical missing link, blending the features of both humans and our evolutionary cousins, the apes, offering evolutionary biologists a glimpse into humanity some 3.2 million years ago and answering a question that had sparked debate since Charles Darwin published The Descent of Man, and Selection in Relation to Sex.  She had a small brain more befitting a chimpanzee, she was only a few feet tall, and her jaw was very apelike, but she had what is known as a “valgus knee,” which meant she walked upright similar to a human being.  The ratio between the length of her thigh and arm bones was 84.6%, almost right in the middle of the 71.8% figure for modern humans and 97.8% for chimpanzees.  Her feet were also flat.  In short, this diminutive creature seamlessly combined traits we would consider human and those we would consider ape or chimpanzee.

Crucially, however, her primary human trait was the ability to walk upright and it appeared to have evolved before changes to the rest of the body, brain, and diet also associated with humanity.  Darwin, the father of evolution and founder of the subject, had predicted this a hundred years before the discovery, writing, “If it be an advantage to man to stand firmly on his feet and to have his hands and arms free, of which, from his pre-eminent success in the battle of life there can be no doubt, then I can see no reason why it should not have been advantageous to the progenitors of man to have become more and more erect or bipedal. They would thus have been better able to defend themselves with stones or clubs, to attack their prey, or otherwise to obtain food. The best built individuals would in the long run have succeeded best, and have survived in larger numbers.”  Darwin himself had no understanding of heredity, genetics, or access to fossils.  The animal kingdom was largely unexplored at that point, making his breakthrough thinking all the more astounding.  The discovery of Lucy was as close to definitive proof of the conclusion that bipedalism came first as one can get with events that occurred millions of years ago, and with it “the progenitors of man became more and more erect, with their hands and arms more and more modified for prehension and other purposes, with their feet and legs at the same time transformed for firm support and progression, endless other changes of structure would have become necessary.”

The question in the search for our earliest ancestors then became:  Who were the earliest bipeds, the first species to walk (mostly) upright?  How long ago did this all-too human of traits arise?  A new analysis published in Nature puts the date millions of years earlier than Lucy walked the earth.  Two decades ago, a fragment of leg bone was discovered in Chad’s Djurab desert dating back seven million years, almost four million years older.  Unlike Lucy, the remains uncovered were very limited and damaged.  There are only blackened bits of the high bone and two arm bones, along with a few teeth and a section of jaw.  The thigh bone in particular is missing both joints.  At first, scientists weren’t even sure all the bones were from the same species, Sahelanthropus tchadensis, but they were intrigued nonetheless because this period is believed to be a critical point in the separation of the human and chimpanzee lineages.  “In most respects it looks like an ape,” explained Daniel Lieberman, a Harvard University paleoanthropologist who didn’t participate in the new study.  Sahelanthropus  has a smaller brain than a chimpanzee, plus the distinctive elongated skull and brow.  The skull is interesting, however, because despite the chimpanzee-like structure, the passage to the spinal cord appears to point downward as it would in an animal that walks upright.  In apes and chimpanzees, it points closer to backwards and the neck sits at a more horizontal angle.

This limited evidence was not seen as conclusive, however.  There was no neck or knee to confirm the suspicion we had uncovered a truly ancient biped, but scientists today can do far more than simply look at a piece of bone and make an educated guess as they might have in Darwin’s day.  We have modern imaging and computer technology to compare it with other finds and current animals at a much deeper and more accurate level.  Jean-Renaud Boisserie of the Université de Poitiers and his colleagues were able to compare these scans with 20 traits of the thigh and arm bones from a broad sampling of modern animals and fossils.  These included chimpanzees, gorillas, orangutans, fossil apes, and early humans.  The goal was to determine if the 7 million year old thigh bone had the external shape, internal structure, and thickness to meet the demands of walking upright.  “It was very challenging also, because the bone has been gnawed by, most probably, a porcupine,” reported Dr. Boisserie. “And yet, a great deal of information was preserved both on external morphology and in internal structures that we accessed through micro CT-scanning.”  The results showed close similarities with upright walking hominids and no shared features with traits found only in apes.  “Hence, it seemed clear to us that the most parsimonious interpretation of these results is that the morphology shared by Sahelanthropus and other hominins reflect their common evolutionary history but also similar locomotor adaptions,” Dr. Boisserie explained.  Dr. Lieberman concurred to a large extent, “They make a compelling case, with these difficult to describe bits of anatomy.  Is it smoking gun evidence on its own? Absolutely not. But in my opinion, in combination with the skull, it should lay to rest the debates about whether this is a biped.”

Others, however, disagree.  A study published a couple of years earlier reached the opposite conclusion, but perhaps that shouldn’t be all that surprising.  Dr. Boisserie and his team would likely agree that Sahelanthropus blends traits of both hominids and apes as one would expect of such potentially early divergence.  It’s believed the creature led a hybrid lifestyle, spending time both in the trees as a chimpanzee and on the ground as an early human.  The environment at the time suggests this was the optimal lifestyle for a potential “missing link.”  The area the creature inhabited was a patchwork of forest and palm groves mixed with grassy regions.  A being that could survive in both would have a huge advantage, and is precisely what one should expect from Darwin’s great theory.  Ultimately, it seems clear to me that bipedalism must have developed long before Lucy, who is believed to have been fully comfortable walking fully upright.   Sahelanthropus seems a likely candidate due to both the time period and the environment.  Whether or not we are descended directly from this creature, or something similar around the same time, is not the point.  We have still peered further back into the past of the evolution of bipedalism, a key occurrence on the long path to humans, than ever before.

Incredibly, Darwin’s Descent of Man, though published in 1871, still provides insight to this day, a testament to one of humanity’s greatest thinkers.  As he put it, “As soon as some ancient member in the great series of the Primates came to be less arboreal, owing to a change in its manner of procuring subsistence, or to some change in the surrounding conditions, its habitual manner of progression would have been modified: and thus it would have been rendered more strictly quadrupedal or bipedal. Baboons frequent hilly and rocky districts, and only from necessity climb high trees; and they have acquired almost the gait of a dog. Man alone has become a biped; and we can, I think, partly see how he has come to assume his erect attitude, which forms one of his most conspicuous characters.”  He continued, “The free use of the arms and hands, partly the cause and partly the result of man’s erect position, appears to have led in an indirect manner to other modifications of structure. The early male forefathers of man were, as previously stated, probably furnished with great canine teeth; but as they gradually acquired the habit of using stones, clubs, or other weapons, for fighting with their enemies or rivals, they would use their jaws and teeth less and less. In this case, the jaws, together with the teeth, would become reduced in size, as we may feel almost sure from innumerable analogous cases…herefore, as the jaws and teeth in man’s progenitors gradually become reduced in size, the adult skull would have come to resemble more and more that of existing man. As we shall hereafter see, a great reduction of the canine teeth in the males would almost certainly affect the teeth of the females through inheritance.  As the various mental faculties gradually developed themselves the brain would almost certainly become larger. No one, I presume, doubts that the large proportion which the size of man’s brain bears to his body, compared to the same proportion in the gorilla or orang, is closely connected with his higher mental powers.”  It’s almost impossible to believe that this multi-million year journey is playing out so close to Darwin’s vision.

Elsewhere in the world of evolution, a different, much more far reaching puzzle might be solved:  The origin of teeth.  For years, scientists have believed teeth evolved from the scales of a fish, but how this process unfolded remained a subject of debate.  Did the outside scales move into the mouth or was there something inside that moved out?  A fossilized sawfish, Ischyrhiza mira, from over 65 million years ago provides a possible answer.  Sawfish species still exist today and are easily recognizable by the spikes around the mouth the creatures use to fight predators and find food, almost as if their mouths are encased in a crazy looking cage.  The spikes themselves, known as rostral denticles, are believed to be modified scales present on the rest of the body.   Modern fish teeth are coated in a complex structure called enameloid, also known as urodentine or vitrodentine, which is similar to the enamel in our own mouths.  Both are calcified and incredibly hard.  No one, however, knows precisely when this substance evolved and scientists who recently analyzed the scales of an ancient sawfish expected to find rostral denticles that were essentially slightly modified scales.  Perhaps a step closer to true teeth, but still far removed. Instead, the structure was remarkably similar to a modern shark’s tooth.  Both are made up of bundles of microcrystals, organized into lines around the surface while more randomly arranged as you get deeper into the tooth.  These layers are interspersed with micro crystals arranged perpendicular to the surface, and the combination of the orientations of the minerals provides strength and stress resistance.  Finding a similar arrangement on an ancient animal strongly suggests we’re looking at a very early tooth, but unfortunately it is impossible to say for sure.  They structure could’ve arose multiple times and taken the same general organization because its used for a similar purpose, a process known as convergent evolution.  Regardless, in the absence of any evidence to the contrary, it seems more likely that the outside-in hypothesis is correct, which would make this discovery a real breakthrough.  Hopefully, we will know more soon.  Either way, it’s Darwin’s world and we’re just living in it.


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