Part 6D: The Two Biggest Leaps in Hominid Evolution

The Science of Evolution

The Evolution of Human Beings

by Ardea Skybreak

Revolutionary Worker #1182, January 12, 2003, posted at

The overall evolutionary process which connects modern human beings to our earliest ape ancestors was marked by a number of important junctures or key "milestones." Probably the most significant of the biological milestones was, first of all, the initial emergence of bipedalism in a line of apes; and, second of all, the significant evolutionary modification-- apparently related to an overall "slowing down" of hominid developmental rates -- which took place a few million years after the first emergence of bipedalism and which resulted in human infants being born in a very undeveloped or dependent state (much more so than infant chimpanzees, for instance) but allowing for a tremendous increase in brain size and a much longer period of post-birth brain development.This in turn made possible the unprecedented capacity for learning which is so characteristic of the human species.

The first of these major evolutionary modifications--the initial emergence of bipedalism-- changed hominids' basic form of locomotion and likely allowed hominids to expand their range into a greater variety of habitats and environments. Bipedalism also--objectively at least--"freed the hands" for purposes other than locomotion, and provided an anatomical basis for such relatively simple things as routine carrying of food or other objects over longer distances and the simple use of unmodified natural materials (such as found stones and sticks) as "tools."

It would be a few million more years before any bipedal hominid species would actually begin to strike stones together to produce sharp flakes and thus make (as opposed to just use) simple stone tools. By objectively freeing the hands from near constant use in locomotion, the very initial emergence of bipedalism in a line of apes had set the stage for this later development. Again, even though the fossil record (so far at least) suggests that hominids didn't get around to making even the simplest stone scrapers and cutters until about 2.4 million years ago (some millions of years after the first emergence of bipedalism), it is worth noting that even chimpanzees, which are physically unable to walk upright for any long periods of time, are nevertheless able to use their hands to carry food for very short distances, to wield branches as clubs to scare off predators, to use stones as "hammers" to crack open nuts, or to strip leaves off twigs to fashion flexible probes used to "fish" edible termites out of their nests. It seems likely therefore that even the earliest and still very ape-like of the fully upright-walking hominids would have made even fuller use of their hands than the non-bipedal species from which they were descended, especially during any times when they were moving about on the ground and away from trees.*

As noted, the second crucial biological milestone on the path to becoming human occurred a few million years after the evolutionary emergence of bipedalism, when a significant change in overall rates of development seems to have emerged in a line of bipedal hominids. It is interesting to note that many evolutionary and developmental biologists feel that many significant changes in life's evolutionary history came about as a result of a relatively simple change in the rate of development of one or more bodily structures in an ancestor population. In the case of the hominids, a general "slowing down" of the anatomical and physiological development seems to have led to a "package" of changes, including changes in the proportions of the arms and legs, changes in the shape of the skull and facial bones, etc.

Probably the most significant of these changes was the fact that infants were now born in a very immature and undeveloped state, requiring a much longer period of parental care of these vulnerable and dependent young. At first glance, one might think that giving birth to very immature and dependent babies requiring long periods of care would be such a disadvantage that natural selection would quickly eliminate such an evolutionary "innovation" from populations in which it appeared. But such a change also would have meant that infants born essentially "prematurely" (relative to the rates of development in preceding species) would continue to grow and develop for a much longer time outside the mother's body--and this included their brains!In the earlier species of bipedal hominids, there had been a pretty strict limit on just how much brain size could increase, simply because an infant with too big a brain could not have passed out of the mother's body without killing her. But if the brain could undergo most of its growth and development after the infant was born, the total hominid brain size could greatly increase.

And this is exactly what happened. At a certain juncture in the evolution of bipedal hominids, one or more species evolved having an overall development pattern that was slower than that of the apes or earlier hominids, but whose brains continued to grow and develop for much longer periods of time after birth. In these later hominids, as well as in our own human species, the average brain could actually triple between birth and maturity (and in modern humans the brain continues to grow and develop for almost 2 full years after a baby is born). Thus, something as relatively simple as a slowing down of the overall maturation process seems to have been all that was needed to make possible an incredible expansion in the size of the brain and post-birth period of brain development in these new hominids. This, in turn, evidently allowed for an incredible increase in the hominid capacity for learning , and for doing so in active interaction with the external natural and social environments.

A Series of Transitions from More Ape-like to More Human Features

When you compare modern apes and human beings, you can distinguish between the features that are more typical of apes and those that are more typical of human beings. And many of these features are preserved in fossils. (Obviously, there are also big differences in language and in overall abilities to understand and do different things which cannot be preserved in fossils, but for now I'm just talking about the kind of differences you might be able to detect in some fossilized bones.) When you know which features are more ape-like and which features are more like those of modern humans, you can study the fossil hominids of different periods in the past few million years and get a sense of when the evolutionary modifications which marked the path to becoming human first took place.

Our bodies are remarkably similar to those of apes in many ways: we have very similar bony parts, organs, and biochemical molecules, including DNA. But modern African apes like gorillas and chimpanzees are unable to stand and walk upright for any length of time, because their skeletons are aligned differently. In apes and other non-human mammals the foramen magnum (the hole at the base of the skull where the spine connects with the head) is located much further back than in humans. To get a sense of this, just look at a dog and feel for the hole at the back of its skull--this is also how an ape's skull connects to its spine.

In humans, however, the foramen magnum hole is right under our skulls, so that our heads basically "balance" on top of our spines. This is a crucial feature--found only in the hominid species--which, along with some other changes in skeletal alignment, allows us to stand and walk fully upright.

Modern humans also have skeletons which are differently proportioned than apes--we have many of the same bony structures, but apes have proportionally longer arms and proportionally shorter legs. Their feet are different too, more suited for grasping tree branches than for walking on the ground. When apes do walk on the ground, they mainly move leaning forward on all fours in a style called "knuckle walking," which most of us have probably had a chance to observe on TV or at the zoo.

Apes also have very different "faces" (those of modern humans are "flatter" overall) and proportionally more massive jaws and teeth that include bigger canines. I won't get into a lot of detail here about tooth shape, size and development patterns, but you should know that fossil teeth are very important in figuring out pathways of hominid evolution. The size, shape, and positioning of the teeth reveal a lot about what a species ate (whether primarily plant foods, meat, or a more wide-ranging omnivorous diet like modern humans) and also helps to determine whether different species of hominids are or are not closely related.

In addition, it has been demonstrated that the patterns of tooth eruption and development (which can be deduced from some hominid fossils) can provide quite a bit of information about those all-important rates of development of their entire bodies. An analysis of this data, combined with calculations of such things as brain size relative to size of the birth passage in a female of the species--can, for instance, provide a good indication of whether a particular species gave birth to young whose development rates were more like those of apes or more like those of modern humans. (For an interesting discussion of this tooth developmental data see, for instance, Richard Leakey's book Origins Reconsidered .)

So, when paleoanthropologists find hominid fossil remains (a skull here, a part of a skeleton there-- or sometimes, when they're really lucky, a number of pieces found together in one spot), they start by determining roughly how old these remains are (through various dating techniques previously discussed in this series). Then they try to figure out whether they're looking at a brand new species never before found or an additional specimen of a species previously described. (And sometimes they discover that more than one hominid species lived in the same general time period.)

Paleontologists then measure and study very specific things, such as the proportions of arm and leg bones; the position of the foramen magnum hole; the shape and size of the skull; the places where jaw muscles used to be attached; the shape of the foot or hand bones (when available--unfortunately, these are rarely found); the size and shape and grinding surfaces of the teeth (and sometimes their pattern of eruption and rate of development); the overall size, sex, and approximate age of the individual when it died; whether it was found next to or in the same general area as any primitive stone tools; whether there are any nearby fossilized remains of animals which may have been prey species eaten by hominids and whose bones may reveal "cut marks" made by stone tools used by some hominids to butcher carcasses.

Meanwhile, other scientists often contribute to the emerging picture of the overall environment in which a hominid fossil lived by studying geological features, ancient soils, plant and animal fossils of the same age, and even fossil pollen granules from the same period, all of which helps to recreate a fuller picture of the habitat in which the hominids lived and died: whether, for instance, the area at the time consisted more of forests, woodland savannas, grassland savannas or a mix of all three; what other animal species (including potential prey species or potential predators of hominids) were around; and also whether there is any evidence of any major environmental changes (such as a drying or cooling trend) taking place during that general period.

Were the Earliest Hominids "Just Apes"? The Significance of the Evolution of Bipedalism on the Road to Becoming Human

A great deal of new information about the sequence of changes in the various hominid lines has come to light in just the last few decades, and especially in just the past few years. By comparing the features of many different species of bipedal hominids, it has become clearer that the ones that are generally thought of as the "early" bipedal hominids--the ones that lived between the first emergence of bipedalism and around 2-1/2 million years ago--still had a lot of ape-like features, despite the fact that they walked upright. They were not all alike, but on the whole they tended to be very short, with proportionately short legs and long arms, much like the apes that swing through trees. Their skulls were generally more stretched out than the more rounded skulls of later hominids and humans; their canine teeth were generally longer, like those of modern apes; and in many (though not all) cases their facial bones "jutted out" in more of an ape-like snout than the flatter faces associated with later hominids and modern humans. Perhaps most strikingly, these early bipedal species had proportionally very small brains, which were more in keeping with the brain sizes of apes than humans.**

So it is true, as others have pointed out, that the so-called "early" hominids were still so ape-like that they really should not be thought of as just some sort of "little humans." But, on the other hand, they were not "just" apes either--they were, after all, bipedal!

Some people seem to downplay the significance of the evolutionary emergence of bipedalism (discounting the notion that "freed hands" could have had any real significance since the early hominids made no stone tools and since the hominid brain didn't expand until much later), and choose instead to emphasize the still ape-like characteristics of early hominids by referring to them as "bipedal apes." This term may be fairly accurate in terms of capturing their appearance , but I feel this is somewhat misleading and that it one-sidedly emphasizes the ape-like features while downplaying what must in fact have been the profound implications of the emergence of upright locomotion and more fully freed hands in these unusual "apes" (or "proto- humans")--features which likely enabled them to expand into a greater variety of environments and engage in significantly new behaviors.

Some years ago, it used to be thought that the emergence of bipedalism was in and of itself such a significant evolutionary development that any bipedal hominid could be thought of as being essentially human. The thinking was that as soon as our earliest hominid ancestors evolved the ability to walk upright on two legs, their hands would have been "freed" for purposes other than locomotion and therefore, the thinking went, these early ancestors would "automatically" have started using their hands for such activities as making tools and weapons, hunting and gathering foods, carrying dependent infants, and so on. It was speculated that this, in turn, must have created an immediate and compelling need for increased intelligence, bigger and more complex brains, more advanced social communication and coordination, and changes in family structure and broader social organization.

Well, all these changes did eventually take place, but certainly not overnight! We now know that all this didn't happen as part of one single package of evolutionary modifications at the time of the first emergence of bipedalism: the early Australopithecines of 3 to 4 million years ago or so walked upright on two legs, but their brains remained small and they don't seem to have made any stone tools. The big leap in brain size, qualitatively extended periods of juvenile immaturity, post-natal brain development, and the cognitive ability to actually design and make even the very simplest of stone tools don't seem to have emerged until at least 2 to 3 million years after the first appearance of bipedal hominids. It is therefore true that bipedalism emerged as an evolutionary novelty long before many of the other attributes that we think of as characteristically human.

But, on the other hand, I think it would be wrong to fail to appreciate just how significant the evolution of bipedalism was in the overall process of "becoming human." Even if the earliest bipedal Australopithecine species didn't necessarily use their hands for tool manufacture and for the capture or butchering of animals (and maybe not even for the gathering and transport of plant foods), right from the time bipedalism first emerged, it is still the case that the evolutionary emergence of the ability to walk on two legs effectively set the stage for those later capabilities.

Just think of it this way: imagine if some non -bipedal ape species in the distant past just happened to spin off a new daughter species which was still incapable of upright walking but whose crucial evolutionary modification was to give birth to young with a slowed-down rate of development, requiring prolonged parental care of infants but also allowing the brain to expand and develop for a much longer time after birth. Such a change might well have allowed for a great increase in the capacity for learning and might also have made possible the development of a greatly expanded repertoire of vocalizations (variety of sounds produced) and even perhaps the emergence of complex language.

But, even if all this had happened, what kind of species would have resulted from just such evolutionary modifications if the hands had not happened to be "freed" for purposes other than locomotion by the earlier evolution of bipedalism? The result might have been something akin to a very intelligent chimpanzee, perhaps, but it probably wouldn't be anything we would think of as distinctly human. It took the combination of two major evolutionary leaps, spread out over millions of years--the evolution of bipedalism, followed some millions of years later by the evolution of a mechanism allowing for unprecedented post-birth brain development and learning capacity--for a line of apes to give rise to the modern human species.

So, again, even though it is important to be careful to emphasize that early Australopithecines were still very ape-like (and not "little humans") and that a number of these early bipedal species may have made relatively limited use of their hands for the first few hundred thousand or even millions of years of bipedalism (perhaps until the brains caught up with the hands, in a sense), I think we will be presenting a distorted view of our origins if we don't also emphasize the fact that the later period of qualitative evolutionary transformations (the ones which led to the substantial increase in brain size and capacity for learning) would not have amounted to nearly as much if this second leap had not taken place among species whose bipedalism already allowed their hands to be used for purposes other than locomotion. Even if the early Australopithecines did not yet have the brain development and mental abilities to make "full use" of their freed hands, so to speak (though they probably did at least as well as modern chimps), the evolution of bipedalism and the objective freeing of the hands marked the first great evolutionary milestone on the road to becoming human. It made it possible for that second qualitative leap in human evolution (the later leap in brain size and post-natal period of brain development) to have the effects that it did --producing a species which gained the capacity to transform itself and its surroundings from then on primarily through cultural innovations and the accumulation and transmission of learned information rather than through biological evolution .

It is worth reflecting on the fact that the difference (in forms of productive activities and social organization) between the first members of our own Homo sapiens species (who maintained themselves as simple nomadic hunter-gatherers for more than 100,000 years before inventing agriculture, a mere 10,000 years ago, and starting to build cities) and modern-day human beings (who build cars and computers and explore outer space and the bottom of the oceans) is primarily a difference in culture : none of this required any further substantial evolutionary changes in the fundamental biology of our bodies. Everything that we do today we do on the basis of the same capacity for learning and for transmitting vast stores of accumulating knowledge across the generations through non-genetic cultural means that has been the evolutionary hallmark of our hominid species from the beginning. This, perhaps more than anything, is what makes us fundamentally human and distinguishes us from all the other species.

But none of this would have been likely to occur unless the second leap of roughly 2 million years ago happened to take place on a base which already included bipedalism and objectively "freed" hands.

So, Are We Just An Accident?

The particular ways the evolution of the hominid line happened to unfold weren't bound to take place. A whole different set of evolutionary modifications could have emerged instead, and the evolutionary road could have taken any number of different twists and turns and never produced human beings at all.

Some people find all this very disturbing. One day recently, I was recounting a little of what we now know about the origins of our planet and solar system, the earliest emergence of life on earth, the subsequent 3 1/2 billion years of evolution of all life forms, and how our own species evolved out of preexisting species--all without the intervention of any external supernatural gods or other spirits--to someone who until now had been unfamiliar with all this. After a time, this person anxiously said to me: "But then, what's the point? What's the purpose of it all?"

But that's just it: there is no particular point! There is no particular special purpose to our existence in the grand scheme of things-- except what we make of it. Whether we're even here or not doesn't really matter (at least not consciously) to anything on this planet except ourselves; and certainly (at least at this point) our existence or non-existence can't possibly have the slightest impact on anything in the greater cosmos, where we are objectively of less significance than a single grain of sand on a beach.

But so what? Does that mean we don't matter?Does it mean that we might as well kill each other off because there's no god out there to care what we do one way or the other? Does it mean that our lives have absolutely no purpose? Of course not! Our lives are precious and we do matter a great each other!

We should decide to "do the right thing"--and act with each other with some integrity and in ways that are "moral and ethical"--not because we're afraid we'll get written up by some warden-like god if we don't, but because what we do directly affects the quality of human life. And, of course, our lives can and do have purpose (though different people will define that in different ways in accordance with their world outlooks), because we humans can choose to imbue our lives with purpose!

So here we are: a bunch of wonderfully complex living creatures, who have been at one and the same time highly destructive and highly creative, with an unprecedented capacity to consciously transform the natural world around us and the societies in which we live. There's nothing else "out there"...but isn't this plenty enough?


* The more we learn about the earliest of the bipedal hominids, the more it seems likely that they may have continued to spend a good deal of time still moving about in trees (and almost certainly resting and sleeping in trees, out of the reach of many predators). They still had the long arm bones of "brachiating" (tree-swinging) apes, and it now appears that bipedalism first emerged at a time when much of Africa was still blanketed by relatively unbroken tropical forests. The old idea that the first bipedal hominids evolved at a time when large stretches of largely treeless grassland savannas had spread across large parts of Africa (and that natural selection might have favored upright walking if it enabled individuals to more readily cross these open expanses, see above the tall grasses, and survive away from trees for days at a time)--is no longer considered very credible, because it turns out that tree habitats were still very plentiful in many parts of Africa at that time. Truly extensive belts of open and relatively treeless grassland savannas did develop, but this apparently occurred quite a bit later than the emergence of the first bipedal hominids. There is, however, evidence that the traditional expanses of unbroken tropical forests were at least beginning to break up into more "patchy" habitats (consisting of a varied "mix" of large and densely forested areas intersected with pockets of more open woodland savanna areas) around the time the first bipedal species seem to have evolved.

Keep in mind, of course, that bipedalism did not in any case evolve "because of" any particular environmental changes--as we discussed earlier in this series, a changed environment does not, in and of itself, "cause" an evolutionary novelty to appear. But an evolutionary novelty which happens to appear--through random genetic reshufflings, etc.--is more likely to be maintained and to spread throughout a population over a number of generations if this population happens to be encountering some environmental changes at that time and if these new evolutionary modifications happen to enable individuals to better survive and reproduce in the face of those changes. Thus, it could be that individuals in populations which happened to evolve a more upright- walking posture and locomotion may have gained a "reproductive edge" simply from being able to spend at least some periods of time away from the trees, and to more readily move back and forth between the trees and the pockets of relatively more open habitat which were beginning to develop. This kind of new behavioral flexibility might very well have enabled them to exploit a wider assortment of plant foods and other resources in the patches of mixed woodland savanna and on the "edge" of the traditional forests. And, even if the earliest bipedal hominids still spent a good deal of time in trees, it is possible that they started to more routinely carry foods from one place to another (rather than always eating food on the spot), including across this wider range of mixed habitats; if so, this could have greatly affected everything from their individual nutrition to the nature of their social interactions (for instance, if individuals in even these earliest bipedal species started carrying food back from afar to share with others in a group).

In any case there is no doubt that, in the same general period when the diversity of non-bipedal ape species was sharply declining , bipedalism emerged and the diversity of bipedal species rapidly began to increase. This in itself is certainly strongly suggestive of the fact that bipedalism must have conferred some kind of distinct reproductive advantages in the changing African environments of that period and therefore been strongly favored by natural selection.

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** The "early" bipedal Australopithecines had an average brain size of about 450cc, which is only a little more than the average brain size (around 400cc) of modern chimpanzees. But the "later" hominids, including the first representatives of the genus Homo , had much bigger brains: Homo rudolfensis , for example, had an average brain size in the 700-900 cc range-- which represented a near doubling of the average Australopithecine brain size--even though their bodies were still small like their Australopithecine predecessors. And, while the standard developmental pattern in apes like chimpanzees is for the brain to double in size between birth and maturity, in hominids starting with Homo erectus (as well as in modern humans) the average brain size triples between birth and maturity. In modern humans, a baby is born with an average brain size of around 385 cc, which then doubles in just the first year of life and eventually grows to an average of about 1350 cc.

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