by Ardea Skybreak
Revolutionary Worker #1180, December 22, 2002, posted at http://rwor.org
We are the New Kid on the Block
It has become increasingly clear that the whole history of our hominid line—the line of species which diverged from our ape ancestors roughly 5 million years ago and which includes all the species that are more closely related to humans than to chimpanzees —contains some notable periods of species diversification (when a number of new and different hominid species evolved out of pre-existing hominid species) as well as some notable periods of species extinction (when one or more hominid species died out, though often only after hundreds of thousands of years of successful existence).
We also know that our own fully modern human species ( Homo sapiens ) is sort of the baby of the hominid bunch, since it evolved out of its immediately preceding ancestor species only around 200,000 years ago. By comparison, the best current scientific estimates of the point at which the first bipedal hominid species initially split off from the ancestral ape line date this event to somewhere in the vicinity of a little more than 5 million years ago.
Of course even this very first step , which came to distinguish our most direct ancestors from all previous species -- the emergence of bipedalism in a line of apes -- is itself a relatively "recent" development in the overall timeline of the evolution of life on this planet: remember that the first signs of primitive bacteria-like life go back some 3-1/2 billion years and that, for example, the first fish evolved some 500 million years ago; the first land-crawling amphibians evolved out of one line of the bony fishes some 400 million years ago; the first reptiles evolved out of amphibian ancestors more than 300 million years ago; and the first mammals, the first birds and the first flowering plants all evolved around 200 million years ago (with the first mammals and the first birds evolving out of two separate lines of reptile ancestors).
Some of the earliest mammal species were small rat-sized creatures that were already around some 150 million years ago, at a time when dinosaurs still roamed the earth. Though there seemed to have been relatively few of these mammal species around at the time of the dinosaurs, some of them did manage to survive the global environmental disruption of 65 million years ago (which was most likely caused by the impact of a large asteroid which hit the earth around that time with an impact equal to many nuclear bombs), a dramatic event which ultimately killed off not only the dominant dinosaurs but also a high proportion of all the other species of plants and animals on earth. But the small and rodent-like species of mammals which survived this event and its aftermath soon underwent a "burst" of expansion and rapid diversification, giving rise to many new mammal species. This burst of diversification is thought by many to be related to the fact that the extinction of the dominant dinosaurs and many other species likely created unprecedented opportunities for the populations of surviving mammals to expand and undergo what is termed an evolutionary "adaptive radiation" -- spreading into the newly open or "available" environmental "niches" which had been vacated by the many species that had gone extinct, and then undergoing a series of further evolutionary modifications in the process of adapting to these new opportunities.
In any case, there is no doubt that the mammal line from that point on continued to evolve and spin off new species for millions of years. It produced the first "anthropoid primates" (the group of mammals which would eventually include all the monkeys, the apes, the hominid ancestors of humans, and humans themselves) starting around 35 million years ago. From their evolutionary origins around that time, the primate line continued evolving -- and further subdividing into all sorts of different species -- for another 30 million years or so before ever spinning off the primate side-branch which would be made up of our most direct ancestors, the upright-walking hominids.
So while life itself began evolving on this planet some 3-1/2 billion years ago, and the primate line as a whole has already been evolving for some 35 million years or so, the upright-walking hominids didn't appear until roughly 5 million years ago, and the most recently evolved of all the hominid species -- our own species Homo sapiens -- really is the new kid on the block, having been around for quite a bit less than one million years.
It is interesting to realize that, around 20 million years ago, at a time when huge swaths of relatively unbroken forests blanketed much of Africa, there used to be many more species of African apes than there are today. But this flowering of numerous forest-dwelling ape species was later greatly reduced to just a handful of species (and today the gorillas and the chimpanzees are the only remaining species of apes on the African continent).
The same kind of "whittling down" of species numbers and diversity seems to have happened to our own hominid line: in the beginning there seems to have been only one or a small handful of bipedal hominid species. Very quickly, however, the number of different bipedal hominid species seems to have started to significantly increase; and then later these bipedal hominid species underwent yet another wave of species diversification, before finally getting "pared down" to just a handful of species and eventually leaving us the one and only hominid species still around.
This kind of process and pattern is actually very commonly observed throughout the natural world after a new evolutionary line emerges out of an ancestral line on the basis of some kind of significant evolutionary modification (such as the emergence of bipedalism in a line of tree-dwelling apes): a period of rapid evolutionary diversification (generating many new species) is often followed by what can be thought of as a sort of "pruning" of the evolutionary bush--a period during which the speciation process seems to slow down and some species go extinct while others maintain themselves in the picture for a while, but without necessarily spinning off any new species.*
The Emergence, and Consolidation, of New Species
As we discussed previously in this series, all significant and substantial evolutionary change (including the kind of smaller scale back and forth evolutionary modifications which are continually occurring inside populations of any one species and which are often referred to as "microevolution") can only take place over many successive generations. And this is all the more true of the process of full speciation - -the initial splitting or "budding off," plus subsequent consolidation, of a brand new (fully distinct and reproductively isolated) species out of a different pre-existing species.
This process may start off with a relatively small bunch of non-typical divergent individuals who find themselves in one way or another reproductively cut off from the larger and more typical ancestral stock populations from which they are derived (and biologists have actually observed this process taking place among living species). But a substantial increase in the numbers of the new oddball population, as well as the further evolutionary amplification and consolidation of the key differences (relative to the ancestral stock), which typically signals that a population of organisms represents a truly new species (as opposed to just a minor or temporary variant of the old one)-- all that is a process which can only take place over a great many reproductive generations.
As was also discussed previously in this series, the start of the whole speciation process may be pretty "sudden" (conceivably starting off with just a handful of genetic mutations or recombinations in just a few individuals for instance), and the later appearance of that new species in the fossil record may also appear to be pretty "sudden" (especially since we would be unlikely to have any chance of finding fossils of the new species until enough time had gone by to allow their numbers and their geographic range to substantially increase!). But the process of consolidation of a speciation event, the point at which you might be able to say, in effect, "Yup, this is definitely a new species, it didn't just go extinct as soon as it emerged, and it didn't just get genetically reabsorbed into its ancestral stock -- it kept its distinct identity, and it looks like it's going to be around for a while" -- that consolidation process takes much longer.It might still be described as relatively "sudden" on an overall geologic time scale (where things are measured in hundreds of thousands and millions of years!), but it wouldn't be surprising to find this taking place over hundreds or thousands of reproductive generations. In a short-lived and fast-reproducing species like fruit flies, it might only take a few years to "bud off" a new species (and in fact this has actually been observed to happen in laboratory populations); but in relatively long-lived and slow-reproducing species like the primates, the accumulation in a population of sufficiently significant evolutionary changes to lead to full speciation could easily be spread out over hundreds or thousands of years.
Key Junctures in the Evolution of Human Beings
I won't repeat here all that was said in the previous installments of this series about the mechanics of how speciation takes place, and about the importance of reproductive isolating mechanisms of various sorts in "getting the speciation ball rolling" so to speak. (For a discussion of these issues see "The Science of Evolution," parts 4a and 4b, RW #1163 and #1164.) But I would like to emphasize once again that the current scientific thinking is that major transformations and disturbances of local environments have likely often played a crucial initiating (or "trigger") role in shaking up an otherwise relatively static evolutionary line of plants and animals and spinning off one or many new species.
I bring this up here in particular because two sets of important environmental changes (which we will discuss a bit later in this series) seem likely to have been involved with spurring on two particularly major junctures or "leaps" in our own human evolutionary history: first , the point at which the first bipedal (upright-walking) hominids diverged from the earlier ape species which did not walk fully upright; and second , the point (some few million years later) when a qualitatively different branch of bipedal hominids spun off from one of those earlier bipedal hominid species (exactly which one is still being worked out) to give rise to a series of species which were much more like modern humans and which are generally classified as belonging to our own genus, Homo .
The various species in the genus Homo , which lived starting around 2 million years ago, generally had much larger brains than apes or "early" bipedal hominids such as the Australopithecines.In fact, the average brain size of the early Homo species was almost double the average brain size of apes or early Australopithecines , even though their average body size was not much different.
But the fossil record shows that the size of the pelvic hole through which babies pass when they are born was too small to allow such large-brained creatures to pass through (and this is the case in both the early species in the genus Homo and our own modern species). So how could the later hominids and modern humans end up with such a big increase in brain size relative to older ancestors? The answer to this seems to be the emergence of a crucial evolutionary modification in the overall rate of hominid growth and development -- a change which essentially resulted in hominids in the genus Homo giving birth to "premature" (or not fully formed) infants, whose brains continued to develop for a long period of time after birth and outside the mother's body, eventually tripling in average size in the period between birth and maturity in the case of the later members of the genus Homo and in our own modern human species. (In modern humans, a baby's brain doubles in size just in the first year after birth.)
It is interesting to note that many evolutionary modifications throughout the history of life have apparently come about through relatively minor "tweakings" of the rate of growth and development of one or more body parts. For instance, since the 1980s, evidence has been accumulating that even small mutations in the handful of genes that control embryonic development (the homeotic genes which turn other genes "on and off") can cause dramatic changes in anatomical organization or "body plan." Some of these kinds of relatively minor "tweakings" of rates of growth and development may produce individuals with new anatomical features which may ultimately prove to be reproductively advantageous and therefore get spread by natural selection over successive generations.
In the case of the upright-walking hominids-- whose form of locomotion and relatively "freer hands" no doubt already allowed them to explore and manipulate their surroundings to a much greater extent than any prior ape species--an evolutionary "tweaking" and slowing down of overall rates of growth and development seems to have represented the second great evolutionary leap and key turning point on the road to becoming fully human. This change led not only to an increase in total brain size (which was considerable), but also to an increase in the length of time during which the brain could grow and develop outside the mother's body . This seems to have been the key development which allowed for a greatly increased capacity for learning , in active interaction with the dynamic outside natural and social environment.
Furthermore, the fact that the infants of the later hominids would have been born (as is the case with human babies) in a very undeveloped, dependent and immature state, meant that adults would have had to take care of them more extensively and for much longer periods of time than had likely been the case in earlier hominids (just like human babies have to be cared for during a much longer period of time after birth than chimp infants, which are much more mobile and independent right from birth). The need for a much longer period of infant care would no doubt also have had important implications for hominid social organization, spurring the development of enhanced social coordination and communication within families and larger groups and facilitating expanded teaching and learning.
Even from that point, it would take some time for fully modern humans to evolve: The fossil record clearly shows that the "early" species in the genus Homo were not as close to fully modern humans (either in their physical features or in their capabilities such as tool-making) as their "later" descendants who gave rise to our own species after roughly 2 million more years of further evolution in the Homo line. The emergence of the evolutionary modification which apparently caused a slowing down of overall growth and development in hominids and allowed for greater post-birth learning and brain development seems to have, in a sense, opened up a period of fine-tuning of what we tend to think of as distinctly human characteristics. Throughout this whole period, among populations of physically vulnerable hominids with highly dependent young, natural selection would likely have greatly favored any increased capacity for learning and for handling the complex challenges posed by the natural and social environment through more flexible behaviors and increased social coordination, including through more developed spoken language and other forms of communication.
It is likely that even some of the earliest bipedal hominid species (such as those generally classified as Australopithecines and similar species) made at least some use of primitive tools (and in fact some degree of tool use to obtain food is not unique to the human line and has been observed repeatedly in chimpanzees, and even in some birds in the crow and jay family). Many of the earliest tools used by hominids may have included natural tools such as those used by modern chimpanzees, who throw branches at threatening predators, strip leaves from twigs to "fish" termites out of their nests, or use stones to crack open nuts.
The earliest bipedal hominids, such as the Australopithecines , likely could do many such things and may also have been beginning to use things like unmodified hollow gourds or scavenged animal bladders and skins to carry things like water or gathered plant foods with their relatively free hands. We don't know for sure, because such unmodified and rapidly decaying materials would be unlikely to be preserved in the fossil record. We do know that the early bipedal hominids did not make any modified stone tools: the first evidences of manufacture of even the simplest stone tools (the simplest stone scrapers and cutters which hominids learned to make by striking certain types of stones together to make sharpened stone flakes) do not appear in the fossil record until about 2.4 million years ago, or a few million years after the first emergence of bipedalism!
It is not until the evolution of the first hominid species belonging to our own genus Homo that we start finding evidence not just of tool use but of the more complex and mentally challenging tasks of tool manufacture . Using unmodified natural materials as tools of sorts is something early hominids may well have done (perhaps even more extensively than chimpanzees do today); but it is much more mentally difficult and challenging (even for modern humans!) to figure out how to strike just the right stones at just the right angles to succeed in making even the simplest of sharp stone cutters and scrapers which were used to butcher animal carcasses and which begin to appear in the fossil record around 2 1/2 millions years ago.
By the time later species of Homo (such as Homo erectus and Homo ergaster ) appear in the fossil record, the "tool culture" had developed even further to include such things as more complex stone axes as well as the conscious and calculated use of fire (which can be used for warmth, cooking tough foods, and protection from predators). Of course, it is not just the physical anatomy, physiological development, and technological abilities of hominids which changed over time. These changes would most likely have been associated with many important changes in their behaviors and social structures as well.
Part of the excitement of studying all the changes that took place in the various hominid species, from the earliest Australopithecines to modern humans, is that their fossilized remains give us clues not only about what they looked like but also about how they lived and what it actually means to "become human."
* The overall effect of such periods of "relative stasis" (when not many new species are being produced) is that the initial burst of species diversification is eventually "trimmed" and reduced over time, at least until a later point when something appears to "trigger" yet another burst of speciation-- producing a new crop of species out of a line which had become, in a sense, rather static and stodgy from an evolutionary standpoint. There is a lot of scientific interest these days in trying to better understand just what factors (or combinations of factors) spur on, facilitate or in various ways contribute to such changes in the overall rhythm and frequency of speciation in specific plant and animal lines, or even in many different lines more or less simultaneously. As scientists make progress in the task of reconstructing the total environmental picture at different times in earth's history and in particular geographic locations (including being able to reconstruct not only what the terrain and climate were like and other features of the physical environment but also what assortment of plant and animal species made up the local biotic environment), what appears to be emerging is that periods of particularly extensive speciation are often associated with periods of significant destabilization or restructuring of the physical and/or biotic environments in which different species lived. Significant changes in external environmental conditions often drive species to extinction, but they can also be favorable conditions for oddball variant populations to get a toe-hold and to give rise to new species having new features that are more in sync with the new external conditions.
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