The idea that there was a now-extinct “missing link” between apes and present-day humans goes back to Darwin’s time, but the idea that it was a single species has long seen as being simplistic. We now know that maybe as many as a dozen species of human lived before the rise of Homo sapiens. But before the appearance of larger brained beings considered to be the first humans, 2.5 mya, there were apes with brains no larger than those of chimps that habitually (i.e. all the time) walked upright rather than on all fours – unlike any ape now living. It is these that are closest to the idea of a missing link – or of the not quite human “man-apes” described by the late Sir Arthur C. Clarke in the novel version of 2001: A Space Odyssey.
A key question to understanding the relationship between these fossil apes, humans and our closest living relatives the chimpanzees is to determine the time of genetic isolation between the latter two. Current estimates vary. Yang (2002) obtained a divergence time of 5.2 mya, with a 95% confidence interval from 4.6 to 6.1 mya. Kumar et al (2005) obtained a 5.0 myr divergence with a 95% confidence interval of 4.4-5.9 mya. However some reject these studies and claim that upright apes lived earlier.
What do you call an Upright Ape?
For many years, the term “Hominid” sufficed. The term was derived from the Linnaean Family Hominidae, which was taken to include modern and extinct humans (Homo sapiens, H. erectus, the Neanderthals, etc) and the upright apes, which were then generally contained in a single genus, Australopithecus. This latter grouping was split into two: “gracile” and “robust” australopithecines. The famous “Lucy” is an example of the former. The great apes (the two chimp species, gorillas and orang-utans) were banished to their own family, Pongidae. The two families were lumped together into a superfamily, Hominoidae, to which the term Hominoid could be applied.
However genetic studies in the mid-1970s showed that chimps were most closely related to humans, followed by gorillas with orang-utans a distant third. This strongly suggested a common African origin for humans, chimps and gorillas but made nonsense of the existing classification. Clearly humans and great apes belonged in the same family, and by the internationally-agreed rules of taxonomy, the more recently-described grouping, Pongidae, was “sunk” or subsumed into Hominidae. This meant the term “Hominid” could now mean a great ape, or indeed anything within the great ape clade, such as Sivapithecus (believed to be ancestral to the orang-utans).
To get round this problem and get back to a grouping that includes humans and only species more closely related to us than chimps, taxonomists have recently begun using the ranking of Tribe (which rather illogically comes below Family in the Linnaean scheme) Hominini, which gives us the term “Hominin”. Some still include the chimps in this grouping, though they are more usually given their own tribe, Panini.
The Hominins, then, include two broad groupings - habitual upright walking (biped) apes, and larger-brained humans. Note that Hominini is a subset of Hominidae: all hominins are also hominids.
Of the thirty or so hominin species that may have existed since the divergence with the chimpanzees, just one remains: Homo sapiens.
Defining features of the Hominins
A hominin is distinguished from other hominids by various adaptations for a terrestrial rather than arboreal lifestyle. They are all habitual bipeds, with the skull sited on top of the vertebral column. Unlike other primates, the feet are not prehensile and lack an opposable big toe. However the opposable thumb is well developed.
In addition to upright walking, the evolution of the hominins is often described in terms of three other new features: reduction of anterior teeth and enlargement of cheek teeth, elaboration of culture and a significant increase in brain size. These features arose at separate intervals, developing at different rates (an example of what is known as mosaic evolution). Tools appear at 2.5 mya; brain expansion does not occur until after 2 mya; but bipedalism is evidenced at 4 mya and may have existed in very first hominins known, which would make it the primary hominin adaptation. In other words even the earliest hominins were bipeds, but they had few other humanlike traits.
Bipedalism in Humans
While the great majority of tetrapods are quadrupeds, habitual bipedalism isn’t uncommon – many dinosaurs were bipedal, all birds (when not in flight or swimming) are and the macropods (kangaroos, wallabies, etc) are. Though other apes can and do walk upright from time to time, the hominins are the only tail-less habitual biped group.
Human bipedalism is a striding gait. The legs alternate between a swing phase and stance phase. During the stance phase, the knee is locked in the extended position, requiring little energy to support it. The femur slopes inward to the knee (valgus angle) and two feet are close to the body’s midline. The body’s centre of gravity doesn’t shift laterally very much during each phase of walking. The strong gluteal abductor muscles prevent the body from toppling over.
By contrast, chimps cannot extend knee joints to produce the straight leg in the stance phase and need to expend more energy to support body (try walking with your knees bent and you will get the idea). The femur does not slope inwards to the knee as much as in humans, the feet are therefore placed well apart and the gluteal abductors are not highly developed. Chimps accordingly waddle and this is exacerbated by their higher centre of gravity.
Chimps are a design compromise between tree-climbing and terrestrial (mainly knuckle walking) where as modern humans are fully adapted for terrestrial living. Earlier hominins retained some arboreal adaptations.
Humans have needed extensive anatomical modifications including:
1) Curved lower spine.
2) Shorter, broader pelvis and angled femur, reorganized musculature.
3) Lengthened lower limbs and enlarged joint surface areas.
4) An extensible knee joint.
5) Platform foot in which enlarged big toe is in line with other toes.
6) A movement of the foramen magnum towards the centre of the basicranium.
On the face of it, one might wonder how what is in effect the re-engineering of the fundamentally horizontal tetrapod bauplan to function in the vertical mode ever happened (see Lewin and Foley, 2003).
In fact it isn’t as dramatic a shift as might be expected. Most primates can sit upright, many can stand unsupported and some can walk upright. Thus the human upright posture is best thought of as an expression of an ancient primate evolutionary trend. The dominant motif has always been an erect body (Napier, 1971). The trend moved through vertical clinging and leaping (prosimians) to quadruped (monkeys, apes) to brachiation in apes. It didn’t involve transforming a true quadruped (e.g. a dog or horse) into a biped.
This does not explain why bipedalism arose.
Origins of Bipedalism
Darwin (Descent of Man, and Selection in Relation to Sex (1871)) and many others believed the characters that mark out humans – intelligence, manual dexterity, tool-making and upright bipedalism – would give an ape an advantage over other apes, and thus they were selected for. But Darwin was misinterpreting his own theory. Features don’t evolve because of what they what they might be able to do in the future. For example, no matter how useful the human brain might have proved in writing Shakespeare’s plays, proposing the Theory of Relativity and piloting jumbo jets, brains didn’t evolve for these purposes. The first bigger-brained hominins couldn’t do any of these things; nevertheless bigger brains evolved and an explanation must be sought in terms of what those earlier larger brains could do rather than what modern humans can do. Thus bipedalism probably didn’t evolve to free up hands for carrying or making things; these were merely useful spin-offs from a feature that evolved for some other reason.
In the 1960s the Man the Hunter theory was popular. Although bipeds are slower and less energy efficient than quadrupeds at top speed, at lower speeds they possess greater stamina, which is useful for tracking and killing prey. The more recent Man the Scavenger picture has the superior biped endurance being useful for following migrating herds and scavenging carcasses. One problem with these theories is that the stone tools needed for exploiting carcasses did not appear until well after bipedalism. Another difficulty is that tooth wear patterns on early hominins suggest they remained predominantly vegetarian until 2.5 mya.
The majority of more recent theories are to varying degrees tied up with the climate change that began around 10-5 mya. A change to cooler dryer conditions promoted grasses at the expense of trees and bushes in lower latitudes. Forest-dwelling creatures declined. The apes that had prospered 17-10 mya were very hard hit and many species became extinct, especially in Eurasia. By 6.5-5.0 mya the Antarctic ice cap was repeatedly draining the Mediterranean, depriving Africa and Eurasia of an important moisture source and accelerating the contraction of the forests. With trees more widely scattered, apes were forced to spend more time on the ground moving between them, and this may have encouraged bipedal locomotion and sparked the emergence of fully-bipedal apes.
Theories fall into five main categories:
1) Improved predator avoidance through seeing farther than a quadruped across open plains.
2) More efficient thermoregulation.
3) Display or warning.
4) A dietary shift, such as seed eating or berry picking.
5) Carrying things.
The “Woman the Gatherer” theory dates to the 1970s. On this picture society was based on females and offspring with males being peripheral. In the more open habitat resulting from the climate change, females had to travel further during foraging and often carried infants. Bipedalism would have been an advantage. Another “carrying things” theory is “Man the Provisioner” proposed in 1981 by Owen Lovejoy. This model has males gathering food and provisioning their partners and their offspring. With the male providing the food, females could breed at shorter intervals, giving them an advantage over other large hominoids. But pair bonding and monogamy is incompatible with the large degree of sexual dimorphism seen in early hominins.
The term sexual dimorphism simply refers to physical differences between the two sexes of a sexually-reproducing species. An extreme example is the angler fish, in which the tiny male attaches itself to the much larger female, and lives out the remainder of its life as a parasite, incapable of independent existence, serving only to fertilise the female.
Among the living primates, a strong correlation between mating strategy and sexual dimorphism has been found. In polygamous species, a male can get ahead of the competition in the access to females stakes by being bigger and more powerful than the other males. In such species, therefore, there is a selective pressure on males to be large. But no such condition applies to the females. Thus, in a polygamous species, males tend to be larger than females. However in monogamous species the males do not have any selective pressure to be large, and therefore tend to be the same size as the females.
The fossil record suggests that early hominins possessed significant sexual dimorphism, making any model assuming monogamy highly unlikely.
There are two theories focussing on posture rather than locomotion. One, due to Nina Jablonski, focuses on hominoid threat displays, where individuals stand erect in aggressive encounters.
Kevin Hunt, on the other hand, noted in field studies of chimps that 80% of bipedal behaviour was related to stationary feeding, and only 4% while walking. He believes that bipedalism was initially a feeding adaptation that only later became a locomotion adaptation. Both these theories suggest standing upright preceded walking bipedally.
However in 1980 Peter Rodman and Henry McHenry at the University of California at Davis suggested bipedalism evolved in response simply to a change in distribution of dietary resources. This explanation is more parsimonious, i.e. involves fewer assumptions.
In the Late Miocene, hominoid dietary resources became more thinly dispersed in some cases requiring a more energy-efficient way of getting about. This theory is based on a couple of simple observations. Firstly, human bipedalism is more energy-efficient than quadrupedalism at walking speeds, albeit less so at high speeds. Secondly, chimps are 50% less energy-efficient than regular quadrupeds on the ground, whether knuckle-walking or moving bipedally. Therefore, as the authors noted, “there was no energetic Rubicon separating hominoid quadrupeds from hominin bipeds”.
For bipedalism to evolve, selective pressure was required. More dispersed and otherwise unreachable food resources provided that pressure. Bipedalism enabled apes to boldly go where no ape had gone before.
The theory is attractive, but contains the hidden assumption that the common ancestor of African apes and humans was a knuckle-walker, which many doubt. Also early hominins differ from modern humans in possessing significant degrees of arboreal adaptation. Their bipedalism would have been less efficient than ours, notes Karen Streudel of the University of Wisconsin.
But Lynne Isbell and Truman Young at University of California at Davies support their colleagues’ theory and extended it, pointing out another strategy would be reduce average daily travel distance by reducing group size (the whole group needs to travel between fewer dispersed sources of food for everybody to satisfy their appetites) hence less travel for the group as a whole. This strategy was adopted by chimps. Thus the theory in its expanded form describes two possible evolutionary trajectories taken by hominids as the effects of climate change were felt.
Many theories have focussed on thermoregulation and the need to reduce exposure to the sun, particularly at noon. Peter Wheeler at the Liverpool John Moores University sees bipedalism as a way of reducing exposure to sun while foraging. Hair loss and sweating are other adaptations. Although the traditional view that early homimins lived in open woodland and savannah is now known to be incorrect (Ardipithecus ramidus and Australopithecus amamensis lived in wooded and possibly forested habitats), the need to keep cool while out in the open moving between the small forest patches could have been a factor.
Recently time and motion studies (Foley) have shown that a hominid would need to spend 60% of its time on terrestrial foraging before it would pay off energetically. This is more a case of “not quite half full” rather than “more than half empty”: it was not necessary to be 100% terrestrial – a hominin with 40% of its time spent in the trees, and living in closed woodlands, could still find it worthwhile to make the switch.
Cameron, D and Groves, C 2004: Bones, Stones and Molecules: “Out of Africa” and Human Origins, Elsevier Academic Press.
Groves, C 1991: A Theory of Human and Primate Evolution, Clarendon Press Oxford.
Klein, R 1999: The Human Career (2nd edition), University of Chicago Press.
Lewin, R and Foley, R 2004: Principles of Human Evolution (2nd edition), Blackwell Science Ltd.
Scarre, C (Ed) 2005: The Human Past, Thames & Hudson, London
© Christopher Seddon 2008