A MODEL OF AUSTRALOPITHECINE RADIATION

A MODEL OF AUSTRALOPITHECINE RADIATION

Living creatures press up against all barriers; they fill every possible niche all the world over. …We see life persistent and intrusive spreading everywhere, insinuating itself, adapting itself, resisting everything, defying everything, surviving everything.

– Sir John Arthur Thomson, 1920

The time is 5.5 million years ago, the place is what will eventually be named the Great Rift Valley in what will be East Africa. Among the many animals and plants, some familiar and some strange, is a small, bipedal ape. There are not many of them, they are still something of a rarity. But they are a tenacious group of apes. As the great forests of the Miocene shrank, breaking up into scattered islands surrounded by the sea of grass, the apes declined in numbers. Monkeys proliferated, and became the dominant primate life form. The apes became relics, a few species surviving in the forests. Except for one species which adapted to a new niche, by giving up the forests and taking up life in the bush and savanna, exploiting the mosaic of environments in the Great Rift Valley.

By 5.5 million years, this ape, this hominid we call an australopithecine, had adapted to the new environment. It would eat anything it could get in its mouth and that did not poison it outright. It was bipedal, efficiently traversing the ground between patches of food, water and security cover. It had long arms for it still exploited the trees for food and protection. It lived in groups, a part of its primate heritage: it was an intensely social animal, and an intelligent one.

Over the next several million years, it increased in numbers and occupied the area from Ethiopia to South Africa, at least this is where its fossil remains have, so far, been found. As they spread southward, through what is now Mozambique, Malawi, Zambia, Zimbabwe and the Union of South Africa, they encountered different environments. The australopithecines avoided the dense tropical rain forest and the arid sands of the Kalahari, keeping to those environments similar to the ones they occupied. The cumulative change, though, was important.

The East African Rift Valley is a landscape rich in small environments–lake, river, marsh, gallery forest, savanna, volcanoes etc. The climate is generally warm to hot,

the days are essentially the same length throughout the year. The only seasonality is because of the rain. Close to the equator, the rain comes at two distinct periods of the year–the long rains and the short rains, separated by a long dry season and a short dry season. Not surprisingly, this has its effects on the flora and fauna of the region. They have had to adapt to extended periods of drought and heat.

The veldt of South Africa is a landscape of large areas of similar environments. The climate is definitely seasonal, not because of the rainfall pattern but because of the increasing distance from the equator. East Africa is equatorial; South Africa is in the temperate latitudes. The days vary in length, short and cool in the winter and long and hot in the summer. The flora and fauna adapted to this by growing in the summer and becoming dormant in the winter. Not all the changes to which the australopithecines had to adapt are due to their increasing geographic range. There were global changes occurring simultaneously. There is evidence that the Pliocene was a period of increasing seasonality, with an increase in the mean temperature differential between the equator and the poles, and a decrease in rainfall, although this may have been partly due to the increasing seasonality that caused rainfall to be concentrated in definite periods, leaving other periods without rain.

The australopithecines adapted and survived for several million years. The earliest known australopithecine from South Africa is Australopithecus africanus, dating in the 2 to 3 million year range. This gracile form was beginning to show the physical changes that would characterize its descendants–A. robustus. It was larger than the earlier East African forms, partly as a response to the cooler temperatures of the veldt. Its cheek teeth were becoming enlarged, also its brain was larger as it relied increasingly on problem-solving ability and memory to survive.

The robust forms had very large molars, reduced anterior teeth and a skull well adapted to chewing (extensive areas for muscle attachment, stronger facial bones, reduced muzzle). More and more plants had adapted to the cool season by developing tubers for energy storage, and the australopithecines had learned to exploit this food source. This is not to say that they ignored everything but vegetable foods. Lizards, insects, infant mammals, grubs, fledglings and carrion would have formed part of the australopithecine diet, along with nuts, fruits, berries, leaves and other edibles such as honey and bone marrow. However, during the winter, the lean season, roots and tubers would have formed the mainstay of the diet, teeth and jaws that could efficiently masticate these foods would have been of great adaptive value. It does not matter much how well you live during the best times of the year if you cannot survive the hard times. The robust australopithecines remained omnivorous, but with adaptations to deal with tough fibrous vegetable matter. The environment in South Africa was largely homogeneous and so was the population of robust australopithecines. They had no serious competition; they were the only hominids (the only apes also) for several million years. Then a more evolved hominid arrived–Homo. Homo came bearing culture, carrying it in a larger brain. The australopithecines were not able to compete. Their behavior was not flexible enough, and they could not, biologically, adapt to a new niche that Homo could not occupy, at least not fast enough. Homo had an adaptive niche broad enough to cover any niche Australopithecus could possibly occupy or adapt to; there was, literally, no room for Australopithecus. He disappeared, extinct.

The situation in East Africa was different from that in South Africa. The landscape was a mosaic of habitats, potential niches. These micro-habitats are in constant flux; the Great Rift Valley was, and is, a tectonically active, dynamic environment. Lakes changed, not just seasonally, but over long spans of time. The lakes changed size, shifted their locations, disappeared, reappeared, became brackish or fresh. Rivers reversed their flow, or dried up. Volcanoes erupted, temporarily or permanently altering vast areas, then the volcanoes eroded away. Forests appeared on wetter escarpments of the evolving rift valley and disappeared when higher hills rose and cut off the moisture. It was a turbulent landscape, although many of the changes were on a time scale such that even tens of generations of a australopithecines would not have noticed any difference.

The oldest known East African australopithecines, dating around 3.4 millions year ago, are the ones called Australopithecus afarensis. These afarines are a more primitive hominid than the gracile forms from South Africa. They are, also, 0.5 to 1 million years older. The afarines may not be ancestral to all the australopithecines, but they probably closely resemble that ancestral stock (the “basal hominid”). In the heterogeneous environment of East Africa, the populations of afarines became a heterogeneous collection of adaptive life styles.

At 2.5 million years ago, we have evidence (in the form of KNM-WT 17000) of a definite trend on the part of one population toward an econiche similar to the one to which the South African robust forms were going to adapt. Possibly the early East African robust forms contributed, genetically, to that trend in South Africa. The South African robusts never developed to the hyper-robust extreme that the East African forms did (by 1.75 million years ago), in the shape of A. boisei. They did not need to; it was only the East African robust forms that had to compete with a significantly different adaptive lifestyle of another australopithecine population.

The hyper-robust australopithecines were at one end of a spectrum of adaptive lifestyles. Possibly, they were spending more time on the savanna and in the bush, exploiting the roots, tubers and other vegetable matter in those environments, getting their water more frequently from streams and waterholes, than from lakes and rivers. Some of their adaptations were learned behaviors, but much of it was physical adaptation, genetic. The evidence is in their teeth and skulls.

At the other end of this spectrum of adaptations, was a population that did not develop the cranio-dental modifications of a more vegetarian lifestyle. This population remained more generalized, more omnivorous in its eating habits–perhaps, because life was easier. Possibly they lived along the large lakes and permanent rivers. They did not have to depend so much on tough, fibrous tubers and roots during the dry seasons and droughts; there was enough other food available. These populations probably lived in larger and more socially active groups than did the hyper-robust forms. There would have been a premium placed on those individuals more socially adept, more quick-witted, more capable of learning. Within this population, behavior patterns common to all the australopithecines were elaborated, intensified, and passed on to the next generation. They became increasingly dependent on learning to adapt to conditions, rather than upon evolving physical adaptations.

In the beginning, it was only a marginally “better” adaptation, at least compared to being physically generalized and not very intelligent. Initially, robust populations were more successful because they were better adapted. Learned behavior could be forgotten or learned incorrectly, tools could be lost, and raw material unobtainable. The teeth, jaws, and muscles were always present. Nonetheless, that odd little population of unspecialized hominids persisted; its adaptations did work, after a fashion and, as time went on, they worked even better. Nut-cracking teeth were useful, but ultimately, they narrowed one’s future options. A hand that (with the coordination of eye and brain) could wield a nut-cracking stone could also wield a meat-slicing flake. The capacity for cultural adaptation was a specialization of unlimited options. When some hominid figured out how to put a sharp edge on a round pebble, the door was opened to those options. The population of physically generalized hominids began to expand their econiche, usurping the econiches of the other australopithecines. The robust australopithecines adapted the only way they could–physically. They became hyper-robust, specializing in a narrow portion of their econiche. But, that only served to stave off their eventual extinction, whereas that generalized hominid, now Homo erectus, spread throughout the Old World land mass, except for those areas too cold to endure without fire. But that would come, too.