Humans are genetically closer to Chimpanzees and Gorillas than they are to any other species (see for example Jones 2000 and Jones 2001). This may indicate all three descend from a single ancestral species or kind. There is a lot of other evidence that, when combined, shows this is extremely likely. Interestingly, many Creationists and Intelligent Design supporters seem prepared to accept evolution has led to species as diverse as ducks, geese and swans. Surely then they should have no problem in accepting species as similar as apes and humans have evolved from a common ancestor or kind.
Before genetic evidence became available scientists assumed the split between humans and African apes was completed tens of millions of years ago. They unconsciously wished to accentuate the difference (Wade 2001). This is a good example of juggling the evidence to fit assumptions, prejudices and preconceptions. The split is now given as being somewhere between seven and a half million years ago to more recently than four million years ago. Recent research may indicate it took up the whole of that time (Patterson et al 2006).
Virtually all scientists and many other people accept we can easily trace our origin back three or four million years to an Australopithecus (“Southern Ape”) of some kind. There is disagreement as to which particular species of Australopithecus or even if we have yet discovered the actual species though. Back beyond the development of the Australopithecus complex there is pretty much a gap in the fossil record until the time of ape genera such as Dryopithecus or Kenyapithecus.
That is, between about four million years ago and about fifteen million years. In fact it has been said all the evidence for Human Evolution between about 10 and 5 million years ago can be fitted into a small box (Henry Gee quoted in Wells 2000). Evolution-deniers usually leave out the part “between about 10 and 5 million years ago” when using this quote.
From three and a half million years ago until today, the amount of fossil evidence increases. If you are actually prepared to look, it indicates an overall gradual change from some kind of generalized Australopithecus species towards modern humans, Homo sapiens. We will begin following this evolution forward to the present soon [Australopithecus]. First we’ll look at where Australopithecus came from.
Fossils recently found in Chad of a forest dwelling species dating from 6 to 7 million years ago should eventually tell us a great deal about the original split between ape and human and help fill the gap in the fossil evidence (Lemonick and Dorfman 2002). The species has been classified as Sahelanthropus tchadensis and it appears to be ancestral to the Australopithecus genus (and so to us).
A few fossils of apes dated to between 4.5 and 6 million years ago that appear to be immediately ancestral to Australopithecus have been found in the first point of the human star (specifically in Gona, Ethiopia). Most specimens have been classified as Ardipithecus ramidus (Walker and Shipman 1996, Tattersall and Schwartz 2000, and Jobling et al 2004). Animal fossils found with them show they lived in forest and their teeth indicate they ate mainly fruit (Stringer and McKie 1996). Because our skulls (and especially teeth) are the hardest part of our body it is usually they that survive longest.
If the genetic evidence that shows humans separated from the other African apes as recently as 3.6 million years ago proves to be correct, it is possible all the African apes descend from various Australopithecus species (Gribbin and Cherfas 2001) or at least from Ardipithecus. Most people wouldn’t accept chimpanzees and gorillas evolved from either of these though. Even the 4.5 million-year-old Ardipithecus skeletal structure shows they had already started evolving towards the human type (Ridley 2000) and it is usually agreed evolution very seldom goes into reverse.
The East African Rift Valley has been geologically very active over the period of human development and sorting out the patterns of change in the region will be very illuminating. We know that until about seven million years ago Africa had a more equable climate than at present. It was covered mostly with semi-open woodland. By five million years ago, the gap between North and South America was closing, and Africa began to tilt as the Rift Valley started opening (Tudge 1996). As a result of these geological events parts of Africa became wetter and other parts got drier.
Genetic studies tend to confirm the gorilla was the first of the three species to split from the ancestral population. The gorilla may have already begun developing in the wetter more densely forested parts of the ancestral ape’s geographic range (Ridley 2000). Therefore the first divergence (to the gorilla) may have been ecological. The arrows around the gorilla zone in map 11 show this zone would simply expand and contract around the Gulf of Guinea with changing climate. In the drier, slightly more open country the population developed into the line that was to lead to both chimpanzee and Australopithecus. The two types of Australopithecus skulls are from Johanson and Edey (1982).
So why the next split, that between the human (or Australopithecus) line and the chimp? It, too, may have been ecological but I suspect this split may actually have been geographic. The two species seem to have each occupied a similar ecological zone. Two separate but similar species cannot occupy the same ecological niche or environment at the same time and in the same place. I haven’t been able to find any information as to where in Africa the chimpanzee evolved but at present they are found mainly near or north of the equator. If they developed in the Savannah zone north of the jungle they would not have been subjected to so much selection pressure.
You can see from map 11 that if the rainforest area expands or contracts, the chimpanzee ancestors can simply move north or south with the edge. A north/south split resulting from the dense rainforest of the Congo Basin spreading as far as the mountains of the Ethiopian Highlands would certainly explain many things about our evolution. Study of the fossils recently found in Chad may eventually illuminate, or even contradict, this assessment though.
As a consequence of all this I can’t give a day and a date for this “First Point”, the original split between the ape and the human lines. Therefore we won’t be able to celebrate the anniversary of the occasion every 100,000 years or so and certainly not every year. But speciation is usually gradual. We can presume the common ancestor of humans and African apes was a bit like an Australopithecus or Ardipithecus and even more like Sahelanthropus. Chimpanzees may indeed be the least changed from the ancestral population and so possibly the common ancestor was like a chimpanzee.
But we can’t say that humans evolved from chimpanzees. For all we know chimpanzees may believe they have evolved from us. Some people believe the unusual way the African apes move on the ground (knuckle-walking) indicates the apes may in fact have evolved from more upright-walking ancestors. Knuckle-walking may just be the obvious way for a brachiating ape (PDF) to move along the ground though. Human upright walking may well have developed from a knuckle-walking ancestor (Jobling et al 2004).
To sum up this story of our origin I would say that between 10 to 15 million years ago conditions were right for a brachiating ape population to move into Africa. With changing conditions this population diversified into a pre-human “superspecies”. In time Australopithecus evolved from this superspecies in the South and East African point.
With changing climate members of the Australopithecus genus would have been subject to much greater selection pressure than were the chimpanzee ancestors. Parts of the human star’s South and East African point were extremely cold during phases of climate cooling. Fossils show that over the generations, as we would expect, the genus changed. There was a gradual adaptation of the pelvis towards upright walking, an increase in height and a gradual flattening of the face. In the early stages the change also included a gradual decrease in the size of canine teeth and, in some species, adaptations of the molars that probably indicate a change from fruit eating, to nut and seed eating.
Early Australopithecus fossils dating from 3 back to 4.5 million years ago are usually classified into at least two species. The earliest ones are Australopithecus anamensis and more recent ones Australopithecus afarensis (the first Australopithecus afarensis fossil found was named “Lucy” after the song “Lucy in the Sky with Diamonds”).
The anatomy of the Australopithecus skeletons does show they still spent part of their life in trees (Wade 2001), presumably for protection or to get their food, and the shape of parts of their brain shows their ability to walk upright was limited (Jones 2000). The discovery at Laetoli in Tanzania, of human-like footprints (PBS video 3m.20s) dated to 3.6 million years ago shows that at least some Australopithecus species in that region was capable of it. But it would make sense the ability to walk upright (like every other human characteristic) developed gradually. As the climate dried the distance between clumps of trees became greater and selection pressure for the ability to walk upright increased.
But Australopithecus was an extremely variable genus (Tudge 1996 and Wade 2001). The fossils found may be the remains of more than just these two species (Tattersall and Schwartz 2000). Certainly the more recent of the two, Australopithecus afarensis, seems to have eventually separated into at least two different types, the robust Australopithecus aethiopicus (the word robust refers to their skull and jaw, especially their back teeth which became especially large) and the much more likely human ancestor Australopithecus africanus. It is confusing many of these names are so similar but Australopithecus afarensis may also have evolved directly to Australopithecus robustus proper and to Homo habilis, almost certainly a human ancestor. Have you got all that? There’s a diagram coming up.
The “robust” branch of Australopithecus may have developed on the margin of the gorilla zone through an ecological split in the Australopithecus population. They appear to have developed in a similar direction to the modern gorilla. Gribbin and Cherfas (2001) even suggest they are in fact ancestors of the gorilla, but this view is not widely held because of changes in their skeletal structure. Today there is a tendency to classify all the “robust” Australopithecus species into a separate genus, Paranthropus. Their development shows they were not involved in the further evolution of humans (Tattersall and Schwartz 2000) and unless they gave rise to the gorilla they eventually died out.
Varieties of Paranthropus were present from well before two million years ago until about 1.2 million years ago, and fossils from possibly even as recently as 700,000 years ago have been found. They lived for a long time alongside other Australopithecus species and therefore must have been separated ecologically, probably by remaining fruit eating. Some scientists think some Paranthropus species may have made stone tools by chipping them but others think not. It is not one of the really bitter debates in anthropology. Many people now consider stone tools were first made by a species that has been called Australopithecus garhi (Lemonick and Dorfman 2002).
A number of evolutionary trees are possible but they mostly look something like this:
Most usual interpretation of the fossil evidence
Alternative interpretations of the evidence
Some people feel there are actually many more species than just these (Tattersall and Schwartz 2000) and we have just met Australopithecus garhi but we’ll keep it relatively simple. How the species are defined and what names they are given are not so important. Besides populations of a species differ from each other in different regions and at different times. Many other species have been named but it seems strange to me so many different species are named in the human line but no one suggests near that many in either the gorilla or chimpanzee lines. In the absence of evidence to the contrary we might assume that they have changed as much as humans have since they separated from us and from each other.
Apart from Homo erectus and Homo sapiens at the bottom right all the species in the diagram were only a little more than 1 metre tall. You can see that most scientists accept Homo sapiens evolved from Homo erectus. In turn you can see that Homo erectus evolved from Australopithecus africanus, either directly or via Homo habilis.
Australopithecus africanus lived from three to two and a half million years ago (Tudge 1996) and showed “an amazing amount of variability” (Tattersall and Schwartz 2000). The evidence hints that, as we would expect, different selection pressures acting on Australopithecus in different points of their star gave rise to different gene pools. The wave theory of evolution tells us the geographic extremities of a population are the most different. Populations of a species in cold climates tend to be bulkier than are populations in warmer areas (Bergmann’s rule(PDF). Populations in colder regions also have shorter limbs. This is called Allen’s rule (Stringer and Gamble 1993). In fact there seems to have been a gradient of types, a cline, of Australopithecus from north to south (Berger 2000). The equatorial, or northern, populations of Australopithecus had small heads with sloping or receding foreheads and were more upright standing with longish legs, almost certainly adaptations to slightly more open tropical conditions. In the south they had developed larger heads, probably as a result of cooler temperatures, but they spent much more time in trees and their legs were relatively short.
As a result different regional populations of Australopithecus show different aspects of human ancestry with no one type showing them all. In “Hybrid Vigour and Inbreeding” [Survival] I mentioned that, in spite of widespread cultural beliefs, theories that new species arise from the expansion of very small populations are flawed. The mystery of which population actually gave rise to Homo habilis and then, via Homo erectus, to us is most easily solved if we consider the possibility the opposite ends of the Australopithecus cline were not quite separate species. Perhaps they were the equivalent of different breeds of dog or cattle, and somehow they were able to meet up and mate with each other and form hybrids.
Apart from the geographic separation there is nothing at all impossible about this. It is not unknown for species as different as baboons and chimpanzees to mate with each other in the wild (Goodall 1990). Of course no offspring are produced in this case. But if, in fact, a hybrid were able to survive and replace previously separate populations it would indicate that some environmental or ecological change led to an expansion of numbers.
This allowed gene flow between northern and southern populations. What could a change of this sort have been? I suggest that the development of stone technology was responsible. The first stone technology is called the “Oldowan” and it developed a little before fossils of Homo habilis appear. Improvements in technology have always led to population expansions in the human species and so the original development of it probably would have. And evidence from Galapagos finches shows that during times of plenty, the boundaries become porous.
Besides, new technology usually leads to environmental and ecological change, and therefore a change in selection pressure. Selection would have worked on any combined population, selecting for useful combinations of characteristics from each population for what were, in effect, new ecological conditions. As a stabilised hybrid developed (“Hybrid Vigour and Inbreeding” [Survival]) any gene disadvantageous to survival would have been bred out, as would the more extreme physical variations. Presumably an inability to work out how to use tools effectively would have been a disadvantage.
Tattersall and Schwartz (2000) write “there is still no satisfactory morphological definition” of Homo habilis. Most other researchers (see especially Klein 1989, Walker and Shipman 1996 and Berger 2000) also emphasise this species’ variability. Some people even separate off another species Homo rudolfensis (Tudge 1996 and Tattersall and Schwartz 2000). Again collectively the fossils from the period look like direct ancestors, this time of Homo erectus. But again most individuals have some characteristics that would actually eliminate them from being ancestors. In fact Homo habilis is probably best regarded as one or more varieties of Australopithecus anyway and could just as validly be called Australopithecus habilis (Berger 2000 and Jobling et al 2004). Perhaps Homo habilis could simply be classified as the maker of the stone tools that appear before Homo erectus evolved.
In spite of our apparent variability many lines of evidence show the modern human population has a surprisingly limited gene pool. We may have been a threatened species at some stage. Perhaps this occurred during the change through Australopithecus to Homo habilis or Homo erectus. Homo erectus was actually much taller than either Homo habilis or Australopithecus and so there must have been a strong selection for height during this change. Many people believe this change was probably a result of movement finally onto fairly open plains. Africa’s climate had yet again become drier with the onset of the latest series of ice ages. Even so clumps of trees would still be scattered through the grassy plains, like islands in the Pacific Ocean. Any group of ancient humans would live comfortably for a while once they had control of a clump of trees. As I said earlier, there was probably also selection for intelligence at the time and so it would have been unfortunate to be born an upright ape with the southern Australopithecus short legs and the northern Australopithecus very little brain.
The population of these human ancestors of two million years ago is largely, if not entirely, confined to the region I’ve called South and East Africa, the first point.
In the last few years fossils of half a dozen individuals, probably also ancestral to humans and dating to the time of Homo habilis (nearly two million years) have been found outside Africa. They lived just south of the Caucasus Mountains in Georgia and are associated with the same Oldowan stone technology. Like all other species on the human line mentioned so far they demonstrate a surprising level of variability (Gore 2002). In this case there seems to be no doubt they were members of a single species though. The region was probably at a geographic extremity of the total population distribution at the time, a point on their star. Their presence suggests Homo erectus ,as narrowly defined these days, may actually have evolved in Asia. From a species such as Homo rudolfensis or Homo habilis that had earlier moved out of Africa.
Some members of this Homo erectus proper then wandered back into Africa where another species Homo ergaster had evolved independently from an Australopithecus of some sort (Wade 2001). I suggest that there they mixed (gene flow again) and members of the hybrid population invented a new Stone Age technology, the “Acheulean”. Then some of them moved back out. The wave theory of genetic, cultural and technological evolution is simple isn’t it?
If we disregard the line that led to the Paranthropus species we find that over time there is on average an overall general decrease in the size of the cheek teeth, an increase in brain size and gradual elimination of extreme physical variations as we move from as far back as Australopithecus afarensis to modern Homo sapiens. It is actually very difficult to define precise boundaries between species and to separate them. Many fossil skulls of Australopithecus africanus, Homo habilis, Homo erectus or several other named species or even some Paranthropus skulls are difficult to assign to a particular species. And there is often disagreement as to which particular species it is. In fact I sometimes think many splitters would like to put each fossil found into a separate species.
Lumpers regard all these different species as being just regional variants of Homo erectus. To and fro movement of genes goes back a long way in the process of our becoming human. Homo erectus may have been well named.
See next :: Human Evolution On Trial - Technology
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