The Later Miocene and Early Pliocene Hominids

The thunderstorm could be seen in the distance. The rain had yet to reach the group, but it would soon be upon them. They had started out earlier that morning from the forests near the lake in search of tubers that they knew to exist within the grass plain, beyond the relative safety of the woodlands. They had moved farther from the forest than expected, and it would soon be night. Clearly the group would have to spend the night in the savanna, and they were now looking for a place of refuge away from the roaming carnivores. The tallest member of the group was the dominant male, who stood around 4'A feet tall. While most individuals walked erect, occasionally some would fall back into a knuckle-walking stance, especially when waiting for others to catch up. Their chimpanzee-like faces were expressive, and occasionally a loud yell would penetrate the stillness of the savanna. The dominant male was impatient with the slowness of the mothers and their infants, who tended to fall behind.

Soon they came across a small ravine slicing through the plain. There were no trees into which they could climb and spend the night, so this would have to do. They could hide from the bands of roaming hyenas and the large predatory big cats. It would also be a relatively comfortable place, away from the winds that were now starting to pick up in strength. Early tomorrow they would be safe back in the forest, but tonight they would all have to risk sleeping in the small ravine. The group started to settle for the night. The mothers and their infants slept in the middle of the group, while the males and older females settled around the periphery. They made little noise, all knowing that their safety depended on not being heard or spotted by the large carnivores that would be patrolling close by. Most went to sleep with the distant sound of rolling thunder.

Some awoke just moments before the catastrophic event, hearing an increasing roar, while around them the walls and floor of the ravine gently shook. Others remained asleep, oblivious to their fate. It had only been a few hours since they had settled for the night, but now a huge and concentrated wall of water was rushing through the ravine, carrying all in its wake. Almost as soon as the flash flood had come, it was gone, and the dry wadi was now a soaking, mud-filled channel. All evidence of the group had been swept away; they all lay washed farther down the ravine and covered deep in meters of mud. The savanna did not notice their passing.

In the mid-1970s, shortly after the discovery of the now famous "Australopithecus" afarensis skeleton, commonly called Lucy, Johanson's team also found a rich fossil bed containing a large number of afarensis fossil remains. They named this fossil site Locality 333. The preserved bones indicated that 13 individuals were present: males, females, and at least four infants. This is the group of fossil specimens that has commonly been referred to as the "first family." The peculiar thing about this fossil bed is that the only fossils found were of the hominids; there was almost no "background noise" of other fossil animals (see Johanson & Edey, 1981). What befell this "family" group to, presumably, kill them all in one catastrophic event and leave their remains to be discovered almost 3.2 million years later at Locality 333? There was no evidence of carnivores that might have eaten them, and no remains were found of the large grazing animals that shared the plains with them. Johanson's explanation seems the most logical: They had been caught in a flash flood while sleeping or resting in a wadi, or dry riverbed. Geological studies of this locality tend to support this hypothesis: The remains are associated with thin clay sediments, suggestive of a sudden event like a flood (Johanson & Edey, 1981). Wadis are notorious for flash flooding; in the badlands, rain from many miles away can suddenly appear from nowhere, hauling down a wadi like a confined mini-tidal wave, carrying away everything in its path. The finding of this group is, of course, significant in the interpretation of the social dynamics of these hominids—if they really were all one social group (which appears likely), then they must have lived in relatively large groups of mixed sexes and ages. What it does not tell us is whether this was a permanent social group, like that of gorillas today, or whether it was a subgroup of a larger community, like those of present-day chimpanzees . . . or humans.

Before commencing our examination of the hominids and hominins, it is necessary to clarify what we mean by these terms. When we refer to a group of fossils as hominids, this means that their phylogenetic status remains obscure, relative to those considered part of the human lineage. That is, they could represent ancestors to orangutans, gorillas, chimpanzees, humans, or none of the above. In other words, they might have their own distinct evolutionary history, not closely tied to the emergence of humans and their immediate ancestors. Hominid is the vernacular from the family Hominidae, which nowadays incorporates all of the living great apes and humans.

The term hominin is used to describe those groups considered to be closely associated with the emergence of the human lineage. This does not necessarily mean they need to be ancestral to humans. Rather, they share a number of derived features with humans; they share an immediate common ancestor to the exclusion of the other hominids. For example, while the species within Paranthropus are not ancestral to Homo, they are nonetheless "hominins." The word is derived from the Hominini, the name of the tribe that includes those forms that are closer to humans than to chimpanzees.

The hominid or hominin status of the taxa to be discussed in the next few chapters will be addressed in Chapter 5, which will present a phylogenetic analysis (evolutionary history) of the late Miocene and Plio-/Pleistocene taxa. For convenience, in the next two chapters we will often refer to species as hominids, which encompasses the subcategory hominin.

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