Studying mammals: food for thought

4 Who were the ancestors of Homo?

Fossil evidence supports Darwin's view that humans and apes evolved from an ape-like ancestor and, furthermore, suggests that the ape line diverged from the Homo line at least five million years ago (Figure 1). From our current knowledge of the fossils available to us, the evolutionary tree in Figure 3 (below) begins at about six million years (6 Ma) ago, with an ape-like creature, identified as a hominine, and named Orrorin tugenensis. You will notice that from this point on the human evolutionary tree is quite ‘bushy’, having a number of branches.

Figure 3: a human evolutionary tree. The dashed lines indicate that evolutionary relationships between species are uncertain. Note the ‘bushiness’ of the tree; the australopithecine line branches into several species of Australopithecus and also to several Homo species

Figure 3: Compiled by Douglas Palmer and Patricia Ash

There was great excitement among the scientific community when fossil remains of Orrorin tugenensis were found, because the limb bones suggested that it was capable of bipedal walking. Coming down to the ground from the trees (terrestriality) and walking upright on two legs (bipedality) are regarded as key events in the evolution of Homo. DA talks about the importance of bipedalism in LoM p. 294; it is a defining feature of Homo, so placing fossil species on or close to the evolutionary line for Homo has to include evidence of features associated with bipedalism.

Question: Referring to your notes on chimpanzee behaviour from Activity 1, describe the technique used by chimpanzees to move about on the ground. If you have completed unit S182_9, compare the technique used by chimpanzees with that used by monkeys.

Chimpanzees are knuckle-walkers when moving on the ground. They use all four limbs for walking, but they do not walk on the palms of their hands as monkeys do – the macaque monkey has a hand that functions in just this way. By contrast, the hands of chimpanzees curl under, so that their weight is supported by their knuckles. Occasionally, chimpanzees stand on two hindlimbs and walk bipedally for short distances, but the gait is slower and less efficient than human walking. As you know from the TV programme, chimpanzees walk bipedally when wading in water (see 26.10–28.10).

Activity 5

Watch the TV programme from 23.08–29.25 and write notes on the walking technique suggested by the Laetoli footprints. Use your notes to explain, in about 100 words, the likely origin of the Laetoli footprints. Focus on the style of walking and the possible number of individuals involved.

The Laetoli footprints are 3.6 million years old and provide spectacular evidence for bipedal walking. There is no evidence of footprints from the forelimbs, or knuckle prints, as you would expect if the prints were made by an ape. Two individuals walked side-by-side in the soft mud, possibly accompanied by a child, walking in the footsteps of the larger individual. Each footprint has a deeper depression where the heel hit the ground; there is no evidence of a gap between the big toe and the remaining toes as would be seen in prints made by an ape that still climbs trees.

The Laetoli footprints were almost certainly made by australopithecines. At least eight species of australopithecines have so far been identified from their fossil remains. Such fossils are restricted to Africa so it appears that they never migrated to other parts of the world. Evidence from their fossilised teeth suggests that their diet was mainly fruit, with some species eating soft fruits and others eating seeds enclosed in hard husks. The length and shape of the Laetoli footprints, and their age, suggest they were made by a species called Australopithecus afarensis. Over 100 fossils of A. afarensis have been found, some quite close to Laetoli. The dates for the fossils range from 3.9 to 2.9 million years old, so the species was around for a considerable period (Figure 3). One of the most famous fossils is a 40% complete skeleton called Lucy found at Hadar, Ethiopia in 1974. Lucy generated a great deal of argument about whether A. afarensis was bipedal or arboreal (a tree climber), and the following activity will provide you with a taste of the issues involved.

Figure 4: skeletons of (a) chimpanzee, (b) Australopithecus and (c) human

Figure 4: Reprinted from Fleagle, J. G. (1987) Primate Adaptation and Evolution, pp. 81 and 107, copyright © 1997, with permission from Elsevier

Figure 5: skulls of (a) chimpanzee, (b) Australopithecus and (c) human

Activity 6

Study Figures 4 and 5 (above) carefully. Information from these figures has been used to compile a summary of anatomical features of the human skeleton in the final column of Table 2. Print the PDF of the table, linked below. Complete the table by filling in the columns for the chimpanzee and Australopithecus afarensis as indicated by the headings for each row. You will need to use a ruler to make measurements of the long bones in the arms and legs and the hands and feet for comparisons of their relative lengths within each skeleton. Look too at the bones of the digits, i.e. of the fingers and toes, termed the phalanges.

Click 'View document' to open Table 2

View document

Check your entries with the completed version of Table 2 below.

Table 2: Comparison of anatomical features of chimpanzee, australopithecine and human skeleton

Feature Chimpanzee Australopithecus afarensis Human
cranium relatively flat relatively flat domed
       
pelvis long and narrow short and broad short and broad
       
hindlimb relatively short compared to arm length relatively short compared to arm length relatively long compared to arm length
       
feet feet longer relative to length of leg feet longer relative to length of leg feet shorter relative to length of leg
       
hand bones (phalanges) long, thin, and curved at ends of digits long, thin and curved long, thin and straight
       
foot bones (phalanges) long and curved long and curved straight
       
skull perched at right angles to vertebral column, eyes point forwards perched on top of vertebral column; eyes point forwards perched on top of vertebral column; eyes point forwards

As you'd expect, researchers who assert that A. afarensis was a bipedal walker focus on those aspects of the fossil skeleton of A. afarensis that support their view. Recall from the TV programme that the Laetoli footprints have the big toe aligned parallel to the other toes, as in humans. Such a structure indicates that the foot of A. afarensis was not used for grasping as in some apes, notably gibbons, but was better adapted as a platform for bipedalism, as in humans. The skull of A. afarensis is perched on top of the vertebral column, as it is in humans, suggesting strongly that Lucy stood upright. Lucy's pelvis resembled that of humans – short and broad – unlike the long narrow pelvis of the ape.

In contrast, supporters of the view that Lucy was arboreal argue that the relatively short hindlegs have proportions more like those of tree-climbing apes. The curved hand and foot bones would provide grip and suggest tree-climbing. The feet are relatively long, compared to the legs, which argues against persistent bipedal walking, in that such movement would require the feet to be lifted quite high off the ground.

Question: Can you conclude from the fossil evidence whether Australopithecus afarensis was arboreal or walked bipedally on the ground?

On the existing evidence, it's impossible to choose between these two habits. Overall, the conclusion from the fossil evidence is that A. afarensis was more bipedal than not when on the ground, but was also likely to be skilled at climbing trees.

So, there now seems the beginnings of a consensus that Australopithecus afarensis was partly arboreal, partly bipedal. Indeed, the environment in which it lived was variable; Laetoli was open savannah, but Hadar, in Ethiopia, was wooded. Although the arguments continue, at least some degree of development of bipedal walking in australopithecines can be viewed as an evolutionary change of crucial importance. More generally, changes in the shape of the pelvis are thought to be important in hominid evolution, linked to the development of a form of locomotion that is more efficient than that of apes.

Question: What are the advantages of bipedal walking suggested by DA in the TV sequence from Activity 5 and in LoM Chapter 10?

A number of advantages are suggested. Walking bipedally enables individuals to carry items: food, stone tools, juveniles [p. 294]. Standing upright helps an individual to wade in water for collecting molluscs [p. 296]. In savannah grassland, standing upright makes it easier to spot a predator from a distance [p. 296]. More significantly, an upright posture reduces the area of the body exposed to the tropical sun's vertical rays, and increases the area of the body exposed to cooling breezes.

A number of other factors come to mind; standing upright helps when picking fruits and nuts from trees. The assertion that human bipedal walking is more economical in terms of energy used per distance travelled than walking on four limbs is an issue that many see as crucial, as is the notion of the additional cooling that standing upright might bring. In contrast to a good many other less fussy mammals, apes and humans – as large and complex species – are obliged to keep their body temperature constant. For an organ such as the brain, an increase in temperature of even a few degrees would be fatal. For the australopithecines in Africa, the additional cooling possible from standing upright could offer considerable advantages. The notion that wading is of evolutionary significance with regard to bipedal walking is more controversial, and we will be looking at this idea more closely in the latter part of Section 5.

Australopithecus afarensis is of interest because many biologists consider this species to be on or close to the evolutionary line for Homo. You'll notice its prominence in the evolutionary tree shown in Figure 3, where A. afarensis is an ancestor for the first species of Homo. This view is by no means universal; indeed, there is a forest of evolutionary trees, with each tree slightly different from the rest. Although A. afarensis shows some adaptations for bipedal walking, as evident in fossil and living Homo species, it did not have another major defining feature of Homo that I want to discuss in the next section – increased brain size in comparison to apes. You'll notice from Figure 5b that A. afarensis has a relatively flat cranium compared to that of Homo, indicative of a modest-sized brain. The domed cranium that is typical of Homo reflects a very significant attribute of our species.