Hominid Dawn

Daybreak. Crimson light floods a Pliocene woodland. This is Afar, Ethiopia as it was 3.5 million years ago.

A troop of Australopithecines clambers up from the dank recesses of a forest cave. These apish creatures are among the earliest representatives of our own evolutionary line. They are members of the Homini subtribe, a taxonomical grouping that includes modern Homo sapiens and our closest living relatives, Chimpanzees.

Molecular studies indicate that the chimpanzee and human lineages diverged almost 7 million years ago. Although it was once posited (in the early 20th century) that our earliest ancestors dwelt in the tropical jungles of Java and in Eastern China, many lines of fossil evidence now support the notion that the first hominins evolved in Eastern (or, more contentiously, in Central) Africa. The latter theory attained almost universal scholarly consensus 60 years ago, following a slew of ancient Austalopithecine finds from South Africa, Ethiopia and Kenya.

The term Australopithecus does not refer to a single species, but to a group of related species characterized by their large cheek teeth and small brain size (that is, relative to the genus Homo). They are typically viewed as ancestral to Homo. Paleoanthropologists believe that at least 6 separate species of Australopithecus shared the African continent during the mid-Pliocene – and this figure is probably a gross underestimate. Like any other successful mammalian lineage, our evolutionary history was characterized by several successive diversifications and adaptive radiations. The idea that human evolution proceeded in a linear fashion, with one hominid species replacing another, continues to be used as some sort of strange heuristic device in Biology textbooks today, but it is roundly refuted by the wealth of fossil evidence to the contrary that anthropologists have uncovered in the last half-century.

The Australopithecines are human-like from the neck down, but their cranial skeletal anatomy is decidedly ape-like.. They bear large canines and powerful jaw muscles (the masseter and temporal muscles), adapted for a diet of fleshy fruits and the occasional meat dish. The jaws project forward as in most Ape species. In terms of brain volume, size and dentition, Australopithecus afarensis is comparable to the modern Chimpanzee. However, it would be a mistake to think of these ancient hominids as prehistoric chimps. For starters, Australopithecines are not knuckle-walkers. Knuckle-walking is a rather unusual form of quadrupedalism in which the animal moves on all fours with the pressure being taken on by the knuckles. It is a derived characteristic that evolved independently amongst Chimpanzees (and separately amongst Gorillas) well after the human-chimp split. Our Australopiths are, infact, capable of bipedal motion. Although they cannot run bipedally as modern humans do (they walked with their hip and knee slightly bent), all the key adaptations for bipedalism are already in place in their anatomy.

Austalopithecus afarensis was also probably unlike chimpanzees in terms of social structure. The social behavior of apes is wildly divergent from species to species, ranging from the solitary frugivory of Orangutans to the presociality of Human beings. Behavior may not fossilize, but we can speculate as to what the daily life of an Australopith might have been like, based on what we know about their dietary patterns and neuroanatomy.

The females proceed to scour the forest floor for fallen fruits and grubs, pausing at intervals to snatch morsels from the boughs above. Unlike later hominin species, Australopithecine forelimbs are longer than their hind-limbs, making them prolific climbers. Their collections are deposited at the mouth of the cave for later consumption. Having been exempted, by virtue of their age, from the ardors of foraging and scavenging, the elders recline leisurely against the massy trunks of baobab trees and bask in the sunrise. Nearby, younglings wrestle with one another in the undergrowth under the careful supervision of their mother. Like in all other nonhuman primates, childhood is brief among australopithecines and infants are already fairly developed at birth.

The air is filled with the hoots and hollers of the homonins. Whilst these crude primate vocalizations are not akin to a modern language, they do serve as labels for different objects (like “snake” or “leopard”) and can help communicate useful information about the outside world. The structure of their larynx and hyoid bone (that attaches to the base of the tongue) seems to preclude the possibility of speech. Like chimpanzees, they can only modify the sounds produced by the larynx by changing the shape of their lips or by altering the size of their oral cavity – they cannot produce the universal vowel sounds that occur in human language.

Further out, large grey silhouettes lurk about in the dappled light, patrolling the perimeter of this leafy refuge – presumably in the hopes of warding off intruders from neighboring clans. These are the males. Fossil evidence suggests that the males were significantly larger than their female counterparts and the pronounced sexual dimorphism of Australopithecine facial features (average canine size and the thickness of the brow ridges differ greatly between the sexes) indicates that competition for mates among males was intense – as it is in many primate societies.

They have caught scent of an abandoned carcass near the forest border. Meat is an energy and nutrient rich food source that will play an increasingly important role in human evolution in the coming ages. Increasing brain size will be accompanied by a radical transition in dietary habits. Our brains account for a whopping 25% of our total energy demands (in contrast to just 10% in Chimpanzees) and, prior to the advent of agriculture, the consumption of animal foods was essential for sustaining this voracious organ.

During the mid-Pliocene, the lush African treescape began to dry out. The deciduous woods of Ethiopia began giving way to open savannah and more recognizably African grassland vistas. The grassland horses of Europe began migrating south into Africa and hardy, arid-adapted antelopes like the Oryx appeared alongside older antelope species. Bipedality will be a tremendous asset to our ancestors as the forests continue to shrink and Australopiths are forced to trek over larger distances in search of sustenance.

The males emerge from the forest depths and advance towards the corpse of an adult Pelorovis – an extinct species of wild cattle that once roamed the wetlands of Africa. The blazing afternoon sun beats down on the apes. Unlike modern humans, these creatures are hirstute (hominids with bare skin would only appear a million years later) and can more easily overheat in hot environments. Bipedal motion makes them a somewhat smaller target for the oppressive heat and allows them to spot oncoming predators lurking in the grass. Far from being fearsome hunters, Australopithecines are but small game on these plains. They must quickly strip as much meat from the bones of this giant ungulate as possible before larger scavengers are attracted by the smell of rotting flesh.

Too late. In a burst of strength, a female Dinofelis leaps out from an adjoining acacia thicket and pounces on one of the males. She sinks her saber-like canines into the ape’s throat. The other hominids scatter towards the safety of the forest, shrieking hysterically. Dinofelis is a medium sized saber-toothed cat that belongs to the same subfamily of felines as the more famous Smilodon fatalis of North America. We will deal more fully with cat evolution in another post.

The Australopiths may be diminutive in both size and cognitive ability in comparison to members of our species, but they bear the earliest seeds of humanity – bipedal motion will herald a suite of new adaptations – not the least of which will include large brain size and a penchant for tool-making. In the next post, I will deal with the origins our own genus Homo and the expansion of Homo erectus throughout the old world.