WHAT'S NEW IN EARLY HUMAN
EVOLUTION
5 TO 1 MILLION YEARS AGO?
by Alison S. Brooks
Anthro Notes, National Museum of Natural History,
Vol. 18, No. 2 Spring 1996
IntroductionWhere do we come from? What did our earliest ancestors look like and how did they behave? In the last ten years, a flood of evidence, accumulating at an increasing rate, suggests new answers to these old questions. Until recently, the hallmarks of "humanness" were thought to have emerged early in human evolution: full bipedalism by 4 million years ago (mya), and, by 2 mya, tools, nuclear families, division of labor by sex, hunting, long periods of childhood and adolescent dependency, and maybe even primitive language. In addition, as recently as five years ago, the family tree itself seemed rather simple and straightforward; the most common model was a tree with only 7 or perhaps 8 species in all, and only one "side branch". Most of the time, the hominid 'niche' was filled by only one species, except between cat 2.6 and 1.3 mya, when related species occupied the "side branch". First there was "Lucy" (Australopithecus afarensis), from about 3.6 to 2.9 mya. Then, there were more "evolved" australopithecines who came in two varieties: the "gracile" type (Australopithecus africanus) and the "robust" type with huge teeth and a bony crest on top of the skull (Australopithecus robustus, A. boisei, and A. aethiopicus). The former group was thought to have evolved into an early form of our own species, Homo, while the latter "side branch" became more and more specialized, lived alongside early Homo for a while (for perhaps as much as a million years) and then died out. Early Homo, in turn, went through a direct progression from H. habilis to H. erectus, to H. sapiens, marked by increasing brain size and decreasing tooth size. Until about 1 mya, Africa, specifically eastern and southern Africa, was the only home of our ancestors, or so it was thought. In the last five years, new finds, new dates, and new analyses have fumed this simple tree into a complex bush, full of unseen connections, dead ends and mysteries. In addition, the bipedalism, bigger brains, omnivorous diets, tool-making, long period of childhood and reaming, indeed the very "humanness" of early humans, have been challenged. The result has been a dramatic upheaval in our conceptions of our past. While the African roots of the family tree have remained firmly fixed, the timing and number of migrations out of that continent have been matters of considerable debate. In addition to the "where," "what," and "when" of human evolution, the "why" has also been challenged. Was it really so dry in Africa 4 mya that our ancestors had to leave the trees for the savanna? Did larger brains evolve so we could make tools?
Oldest FindsNew finds from two regions have greatly expanded our knowledge of human evolution "B.L." (before Lucy). The first finds, announced in the fall of 1994, come from the Middle Awash region of Ethiopia, just south of Hadar where Lucy herself was found. Here, Tim White, Berhane Asfaw, and an international team of experts found the scattered and highly fragmentary remains of 16 small creatures with large molar teeth, slightly reduced canines, and a positioning of the skull on the vertebral column (backbone) suggesting upright posture. These features suggested human ancestry and an initial placement in the genus Australopithecus. Enough differences exist, however, for these fossils to be placed in a new species, A. ramidus (or "root" in Afar, the local language). For example, the enamel on the canines and molars is relatively thin, the canines relatively large for hominids, and the molars--especially the lower first deciduous or 'baby' molar--smaller than those of other Australopithecus and more elongated than square in shape. The skull opening for the ear was small as in apes rather than large as in Homo and Australopithecus. The leader of the geological team, Giday WoldeGabriel, argues that the fossils are close to 4.4 mya, as far back in time from the actual Lucy find (3.18 mya) as Lucy herself was from the original Homo habilis at Olduvai Gorge (1.9 mya). While the teeth relate ramidus clearly to humans, the limb bones remain to be described. In recognition of the dental differences, White et al. recently suggested that the fossils also be placed in a new genus: Ardipithecus ramidus rather than Australopithecus. White has continued to work in the Middle Awash in 1994, 1995 and 1996, and has announced the recovery of at least one and perhaps several partial skeletons of different individuals. One of the most interesting features of the ramidus find is the apparent absence of a savanna environment, at least in the immediate vicinity. The animal bones and plant remains reflect a forest with colobus monkeys, kudus, bats, a primitive bear, and a number of small mammals but relatively few large savanna mammals such as giraffes, hippos, elephants, rhinos, or primitive horses. In 1995, palaeontologist Meave Leakey and colleagues also announced a new species from cat 4.1 mya, this one from several localities around Lake Turkana. Called Australopithecus anamensis (after 'anam' or 'lake' in the Turkana language), it was differentiated from afarensis because the lower canines were larger, the lower front premolars more asymmetrical, the molars more sloping towards their crowns, the chin region a different shape, and the earhole small as in ramidus. On the other hand, it was distinguished from ramidus by the thicker tooth enamel, larger molars and squarer molar shape. From the asymmetry and angle of the upper part of the shin bone in the region of the knee, however, this form was clearly bipedal. Bipedal knees are quite distinctive because they are shaped so as to allow you to lock ("hyperextend") your knees "straight" while standing and to balance easily over one leg while stepping out with the other. (It was just such a knee joint that led to the finding of Lucy in 1974.) The environment of A. anamensis was less densely forested than that of ramidus, closer to the open savanna envisioned in the earlier scenarios. (More on these early hominids.)
Other AspectsWhich of these two led to Australopithecus afarensis and thence to Homo habilis? This may be a moot question, as Australopithecus afarensis and Homo habilis themselves are challenged as single species stages on the road to modern humans. Is there more than one variant of Lucy, like the multiple species of monkeys and of chimpanzees that co-exist in Africa today? A recent find of a much larger hominid (ca. 24-25% larger than Lucy) at Hadar was interpreted by Kimbel, Johanson and Rak as a male afarensis, but could Lucy's son or brother really have been so different? Or are there two different species of Australopithecus at this time as well? A recent argument by Richmond and Jungers for multiple hominid species in the time range of Lucy suggests that new studies of Lucy's pelvic anatomy indicate that she was actually a "he". Two males of very different sizes would certainly argue for at least two species. Kimbel et al., however, contend that Lucy's pelvic shape is due to her posture while walking and not to an incorrect determination of her sex. Differences in limb anatomy could mean not different species but simply that the heavier males spent more time on the ground while females spent more time in the trees. Furthermore, they argue, not only do all the fossils attributed to afarensis belong in a single species, but the species lasted unchanged for almost a million years. This conclusion is based on comparisons between a new almost complete skull from Hadar at 3.0 mya and a new frontal (forehead and brow regions) from Belohdelie in the Middle Awash region just south of Hadar, dated to 3.9 mya. If early australopithecines were not restricted to savanna environments, were they confined to east Africa? A recent paper describes a new fossil from Bahr el Ghazal in the west African country of Chad, more than 1500 miles west of the east African rift valley sites. The fossil mandible is comparable to afarensis but with thinner tooth enamel and other distinctive traits so it could represent another new species. It is dated to around 3.0 to 3.4 mya on the basis of the primitive elephants, horses, pigs, hippos and rhinos found with it. These are interpreted as indicating a mixed forest and woodland with some grassy areas, rather than an open savanna. Further exploration will probably expand both the range of the ancestral hominids and their variety. The first australopithecus find in 1924 consisted of a child's face, brain cast, and mandible from the South African site of Taung (Australopithecus africanus). The first recognition of different robust (r) and gracile (g) australopithecine species was also based on South African sites: Sterkfontein (g) and Kromdraai (r) in the 1930s, Makapan (g) and Swartkrans (r) in the 1 940s. In recent years, although work continued at these four sites, the main action appeared to have shifted to east Africa, where periodic volcanic eruptions and rift valley sedimentation allowed palaeoanthropologists to find and date actual surfaces where australopithecines had lived. Dates and ancient landscapes were much harder to reconstruct in the cave sites of South Africa. Also, for much of the 1980s and early 1990s, South Africa was isolated from the rest of the scientific community for political reasons. This year, South Africa is suddenly in the early human news again with new finds that shift the picture of human evolution. There are two new fossil sites: Gladysvale and another site as yet unpublished, each with a new series of human remains. Sterkfontein, the first site to yield an adult australopithecine of the gracile variety ("Mrs. Ples"), now contains evidence that more robust forms were there as well at the same time. And at least some of these human ancestors may have been able to hold onto things (like tree branches) with their feet. Newly published foot bones from one of the oldest levels (member 2 ="level") at Sterkfontein, comparable in age to Lucy, show a big toe that stuck out at a slight angle to the other toes. Were there any trees to hold onto? New paleobotanical studies at Sterkfontein from the main australopithecine level (member 4) recovered fossilized vines or lianas (the kind that Tarzan swings on in the old movies) that today occur only well inside the tropical forest far to the north. No open savannas here either! In addition, the younger horizon at Sterkfontein (member 5) has now yielded Oldowan tools dated to about 2.0 million years, slightly older than Olduvai and more "primitive" in their manufacture. Sterkfontein's archaeologist, Kathy Kuman, has suggested most of them were made by smashing quartz cobbles on a hard surface and picking out the good flakes. Who made these tools? Sterkfontein yielded another hominid, younger than the tools and provisionally classified as Homo, but Ferguson has suggested that it may be too robust for Homo and might possibly be reclassified with Australopithecus. Was Mrs. Ples, who comes from the underlying horizon dating to 2.5-3.0 mya (or someone like her but slightly later) the toolmaker? Since gracile australopithecines were supposed to have been ancestral to Homo, while the robust forms were on a side branch, many scenarios had the late gracile forms experimenting with tools, despite the absence of any evidence for tools in gracile sites. (Tools do occur with later robust forms!) Like Lucy, Mrs. Ples may soon undergo a sex change operation and, at the very least, assume a new identity. A recent careful examination of the top of her skull suggested that something was missing. Fortunately, the piece of rock that once encased her skull had been saved. Stuck into this rock were the remnants of a small sagittal crest. Gracile females did not have this feature. Either "Mrs. Ples" was really "Mr. Ples" or else she is one of the earlier members of the South African robust line. At this point, the taxonomy becomes really confusing. If the Homo from Sterkfontein is really Australopithecus, and if the type fossil of an adult Australopithecus africanus is really a robust form like Australopithecus (or Paranthropus) robustus, then who is Australopithecus africanus anyway? Since the original A. africanus was a child's skull and braincase, we really have no way of knowing exactly what it would have looked like when it grew up. There will certainly be many years of arguments before these and other queries surrounding the fossils we now have are resolved, let alone the questions raised by new finds. One postscript to the Taung story involves a fascinating bit of detective work. Most South African sites consist of remains of the lairs of predators who ate australopithecines for dinner, as suggested by the many carnivore tooth marks on hominid skulls and bones. Taung was always different from the others. Despite the mining of what was probably the entire cave, only the three pieces of the Taung "baby" were recovered. No larger or more complete fossils of anything ever turned up. The damage on the Taung skull was also different -- sharp triangular nicks on the edges of the bone, and a distinctive dent in the top of the skull where the thin cranial bone was pushed into the brain. What could have made this damage? Ron Clarke and Lee Berger studied damage from many different types of carnivores and concluded that the only possible agent of destruction was a large eagle, whose talons poked a hole in the skull, and whose curved beak took distinctive bites out of the bone. This would explain why no australopithecine (or other large mammal) adults ever turned up there -they were too big for an eagle to carry. The early evolution of our own species was also once thought to be a simple affair. Tool making, an enlarged brain and smaller teeth marked the emergence of Homo habilis at 1.9 mya. These features were functionally linked together by reasoning that teeth could not be smaller on a larger creature unless some "food-processing" was done outside the mouth, i.e. with tools. By 1.5 mya, even larger brains and modern body size marked the appearance of Homo erectus, who subsequently spread out of Africa. Finally, by about 500,000 years ago, early forms referred to as "archaic" Homo sapiens appeared in both Europe and Africa. The number of species suggested for our own genus has also increased recently, and the relations between them have grown more complicated. What used to be called Homo habilis is divided into at least two, and possibly three, species, while the early Homo erectus fossils from Africa are sometimes put in their own species, Homo ergaster. In the later stages of Homo, once all grouped in the species sapiens, some authors place the early "archaics" in a separate species, "Homo heidelbergensis." and may further delineate the later Neanderthals as "Homo neanderthalensis." The species designation "sapiens" is reserved by these authors for modern humans only. Were all of these groups separate species that could not interbreed and had different adaptations? Did our previous "single species" view of the evolution of Homo obscure what was really happening? Within a few years of finding the original Homo habilis at Olduvai, a very different early form had turned up to the north at east Turkana. This form, dated to the same time, had a larger brain but retained rather large teeth. The Olduvai fossils had small teeth, but brain sizes only slightly bigger than those of australopithecines. Bernard Wood has argued for the name "Homo rudolfensis" (after the old name for Lake Turkana) for the larger-brained Turkana form, and retains the name habilis for the smaller form, whose skeleton, recovered in 1985, suggests Lucy-like proportions of arms and legs. How did these two differ in their behavior? The record is not yet complete enough to tell. Both usedsimple stone tools and occur in the same kinds of environments, usually more open and grassy than those prevailing before 2.5 mya. The difference is not due to geographical separation; a very early example of rudolfensis dating to over 2.0 mya was reported in 1993 from the Malawi sector of the east African rift, well to the south of Olduvai. Which one led to modern humans? This, too, is unclear, and may never be determinable if new early species continue to be found. Perhaps more detailed environmental and behavioral studies now underway will reveal some answers.