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.
EVOLUTION IS REAL
But let’s be clear: This is not evolution versus God. The existence of God—any sort of god, personal or abstract, immanent or distant—is not what Darwin’s evolutionary theory challenges. What it challenges is the challenge of the supposed godliness of Man—the conviction that we above all other life forms are spiritually elevated, divinely favored, possessed of an immaterial and immortal essence, such that we have special prospects for eternity, special status in the expectations of God, special rights and responsibilities on Earth. That’s where Darwin runs afoul of Christianity, Judaism, Islam, and probably most other religions on the planet.
—David Quammen, The Reluctant Mr. Darwin, pp.208-209
As I was growing up I believed that God had created the world. Not in the six days and in the order of Genesis, but that the world existed because of God. I know that I did not believe that the world had been created exactly as Genesis described (either of the two chapters, Genesis 1 or Genesis 2, they’re not the same). The Bible was a book, the stories in it were like Aesop’s Fables, B’rer Rabbit stories, the Grimm’s Fairy Tales, stories, legends, myths, made by Man to tell a story to illustrate a point, a moral, or just for fun.
It was a surprise to me in 10th grade history when Jesus was presented as an actual, an historical person. It was also in the 10th grade biology class though that evolution was presented to me. I first thought that evolution was a metaphor to help explain and understand the biological world. If we looked at animals as if they had evolved from simple one-celled organisms into vertebrates, amphibians, reptiles to mammals we could gain a better understanding of biology. That scientists meant that simple one-celled organisms had actually evolved, that there was a direct, physical, genetic relationship between species was surprising when I realized it. That evolution was not a “metaphor,” that it was meant to have really happened.
Evolution, or as Darwin called it “descent with modification,” is true. It can be seen, measured, and described. Populations of organisms reproduce and change. This change is not necessarily observable, physical change, but a change in the frequency of the genes in the population’s gene pool. The change may result in an observable difference in the physical characteristics of the individuals in the population.
You can look around yourself at all the different breeds of dogs, cats, horses, cows, pigs, chickens, sheep, goats, and the varieties of apples, pears, corn, tomatoes, potatoes, peas, etc. to see the changes between these domesticated species and their wild ancestors, or just between the breeds and varieties of the same species. These examples, you may argue, are examples of human-caused evolution— artificial selection— and not evidence of evolution. I think you are wrong that this matters. It is still descent with modification. Man has only sped up the rate of change and controlled its direction. However, natural examples are the new strains of influenza, antibiotic resistant “germs,” pesticide-resistant insects (man many have deliberately introduced antibiotics and pesticides, but he did not knowingly or willing select for resistant forms, that was a natural result).
The Grants, Peter and Rosemary, have spent years with their research teams, studying several species of finches on an island in the Galápagos archipelago (see Jonathan Weiner’s 1994 The Beak of the Finch). By measuring the length, width, and depth of the beaks of the finches living on the one island, knowing who survived and who bred, and correlating that data with the data on seed production and climate (rainfall mostly) they demonstrated that the size of the beaks changed from year-to-year, the evolution of beak size. The size changed in response to the climate’s effect on the seed crop (size, hardness, number), beak size tracked the climate change. Not only did the population of finches evolve (genetic traits changed from one generation to the next), but the traits changed in a predictable way in response to some external factor. Not only did this research demonstrate evolution but it also demonstrated evolution by natural selection.
Jonathan Wells (in his Icons of Evolution) points out that the beak size oscillated back-and-forth, from smaller to larger and back to smaller. He claims that this doesn’t show evolution at all, apparently because there was not a net change in a direction. He can claim it, however that doesn’t mean he is right. The trait (beak size) does oscillate back-and-forth. The climate oscillated between arid and drought. It did not change in one direction and stay there. Beak size did evolve, evolution does not mean that the change has to be in one direction and has to stay there. Evolution only requires that there be a change from one generation to another (actually a change in the population over some period of time).
The word “evolution” gets used in three different ways. The first way—change in gene frequency of a population over time—is true as discussed above. The second way, as in “the evolution of the horse,” is about the historical pathway that the evolution of a particular species took, its “phylogeny.” That species evolved in a particular way, Equus from Hyracotherium, birds from a theropod dinosaur or from an earlier reptile, may still be open to discussion and not certainly known. The third use of the term evolution refers to its cause. Although Creationists and others may use “Darwinism” as a synonym for evolution and may believe that Darwin invented evolution, the words are not synonyms. The idea of evolution was around long before Darwin (well Charles anyway, his grandfather Erasmus might have a better claim to inventing evolution). Darwin’s contribution was for postulating a workable cause for evolution—natural selection. By proposing a workable cause Charles Darwin made evolution (the first meaning) scientifically respectable.
Others have postulated other causes of evolution—saltationism, random walk, elan vital, punctuated equilibrium, to name a few. Evolution by natural selection may be shown to be not true, or more likely not the whole story. This is a separate issue to the reality of evolution. That is why it is possible to say that evolution is a fact, that evolution by natural selection is a hypothesis.
Evolution, inheritable change, is a fact. It has occurred in populations over time. It doesn’t matter whether you believe it or don’t, change happens. Evolution happens.
A (SCIENTIFIC) CREATION STORY
In the beginning was the void. No space. no time, no matter. God created a small ball of energy, containing all the future Universe. This concentration of energy began to expand, expand “explosively,” at the speed of light, forming space, matter, light, and time. As the Universe expanded it began to cool, matter coalesced into subatomic particles, into atomic particles. These, spun off into the expanding Universe, gathered into clumps, some immense, some small, and many in-between. Clumps gathered with other clumps, some merging into one clump, others rotating around each other. They began to form stars, star systems, galaxies, and clusters of galaxies.
In the interior of the stars, hydrogen fused with hydrogen, releasing heat and light, forming helium. Hydrogen fused with helium, helium with helium, and so on, forming heavier elements. Some stars just burned out, others exploded as nova, hurling hydrogen, helium, lithium, etc. into space. New stars formed incorporating these new elements, along with hydrogen, and continued the process forming even heavier elements—iron, nickel, silicon, besides carbon, nitrogen, and oxygen. Some of these stars also exploded in novae, hurling masses of heavy elements into space.
Now, as God knew it would, stellar formation took a new step. Before some of the stars had had smaller, sub-stellar bodies orbiting about them. These had been gaseous bodies, mostly of hydrogen. Now some of these bodies had rocky cores of carbon, nickel, iron, silicon, etc. On at least one of these rocky planets that was not too far from its sun, or too close, that was not too small or too large, things happened. Iron and nickel settled to the core, lighter elements like silicon floated above. A smaller rocky planet crashed into the surface and a large chunk splashed off but was unable to escape the gravitational attraction of the larger mass and went into orbit about it. The tidal forces of the orbiting moon helped keep the planet’s surface a dynamic environment, also it churned the waters of the ocean in a great tidal wave that continuously circles the planet. Hydrogen and oxygen formed water. Some hydrogen escaped from the planet but it was large enough to keep its nitrogen (and when it formed free oxygen). An atmosphere formed above the rocky surface. Water condensed into rain. Internal heat from gravitational contraction and radioactive decay heated the interior of the planet. Convective columns of molten rock carried the heat to the surface, sometimes the magma broke through to the surface as massive lava flows or volcanoes. Great plates of basalt formed, covering the surface of the planet. Granite islands floated on some. The churning of the convective columns pushed and shoved these plates about, rain falling on them caused erosion. Mountains rose, oceans formed. Mountains disappeared, the oceans expanded and shrank. Elements were buried and then exposed. The planet’s internal heat and the energy from the young star drove chemical reactions on this rocky planet.
Atoms formed molecules of many kinds. Chemical reactions became more varied and complex. Some molecules were able to sustain themselves for a while, some created a local environment that increased the length of time it could sustain itself. Becoming self-sustaining it became alive.
This new life began to evolve, diversifying into many forms, many more complex, some simpler, but ever growing, multiplying, changing form. About 12 billion years after the beginning, one of these life forms became self aware. Then the day came when it looked around itself and saw how amazing the world it lived in was and asked itself “How?” It explored its world, asked questions, and its knowledge grew. God saw and was pleased. It was amazing how everything had grown from that primal ball of energy, all on its own. And God saw and said it was good.