The Human Past (ii)

Second post in the series about the book, the first is here. I’ll complete my treatment of chapter two in this post:

i. “A rich fossil record documenting the earliest hominins has now been discovered on the African continent, with perhaps a dozen hominin species identified as existing prior to 1.5 million years ago […] The fossil finds were initially concentrated in South Africa, but in more recent years significant discoveries have extended to eastern Africa, and even further afield, in central Africa and the Sahara […] New discoveries have also expanded the known time depth of the hominin lineage, to perhaps as much as 6 million years. […]
Identification of early hominins that branched off since the last common ancestor of humans and African apes is usually based on one of two criteria: either (1) postcranial (referring to the skeleton below the skull) evidence of bipedality; or (2) derived dental characteristics that are shared with later hominins but not with apes. Prior to 4 million years ago, there is tantalizing evidence of this early stage of proto-human evolution in the discovery of pre-Australopithecus (“southern ape man”) fossils in East Africa and the Sahara.
The fossil finds from Chad, Sahelanthropus tchadensis, include a nearly complete cranium dating to between 7 and 6 million years ago, as well as fragments of lower jaws and some teeth. […] Between 6 and 4 million years ago, new fossil forms that have been designated as early hominins (as they show a number of hominin features even while retaining ape-like characteristics), have been found in East Africa and the Middle Awash region of Ethiopia.”

ii. “The hominin fossil record becomes much better represented from around 4 million years ago, with the appearance of early australopithecines. A number of australopithecine species then appear over the ensuing 3 million years, with fossils attributed to the genus Homo appearing fairly late on the African scene, around 2 million years ago or slightly earlier.”

In the book the author of the chapter makes a distinction between ‘gracile australopithecines’ and ‘robust -ll-‘. In fact it seems that there is some disagreement whether the two ‘types’ of australopithecines even belong to the same genus, but that a branching process took place in this timeframe (3-1.8 mya) is beyond doubt. Applying the gracile/robust framework, it seems likely that we, Homo Sapiens, are descendants of some form(/s) of the gracile australopithecines, whereas on the other hand the robust australopithecine branch went extinct about 1 million years ago. The early homo species didn’t really have ‘huge brains’ or anything like that, the brains were somewhat/significantly larger than that of the robust australopithecines but compared to our huge brains they are quite similar. Brain development was a gradual process and it is a very wrong way to think about the evolutionary process eventually leading to us, Homo Sapiens, as some sort of narrative about how much smarter species of apes outcompeted the ‘slow’ ones very fast and took over for a while, until some even smarter ones turned up and outcompeted the new dullards – or worse, like a process where a single species just got smarter and smarter over time. Many species of honinins lived simultaneously, and some of the species that eventually died out were around for a very long time – A. boisei were around for almost a million years, half of that time contemporaneously with the early homo species. Also, there were lots of different species around at the same time: “Between 2.5 and 1.5 million years [ago], a number of hominin species (perhaps eight) are found in Africa, including the “robust” large, cheek-toothed australopithecines, as well as more “gracile” forms, and Homo Habilis, H. rudolfensis and H. ergaster/erectus.” When it comes to brain size, robust australopithecines like A. boisei (2.3-1.4 mya) and A. robustus (1.8 to perhaps 1 mya) had cranial capacities of 500-550 cc, whereas “the cranial capacity of homo habilis [an early homo species] ranges between 510 and 687 cc.” – to compare, “the average for modern humans is about 1350 cc and around 450 for chimpanzees and gorillas”. There was still a long way to go when the first Homo species showed up. The book sums some of all this up like this:

“To recapitulate: the earliest bipedal hominins [2.10] appear to emerge in the fossil record by 6 million years ago. During the Plio-Pleistocene, a major bifurcation in the hominin lineage led to the robust australopithecines as one evolutionary branch, or clade, and to the genus Homo as the other. In addition to bipedality, other hominin traits that emerged included longer legs, shorter arms, more dextrous hands with a longer thumb, reduced canines and incisors (and in early Homo, reduced molars and premolars), and brain expansion, again especially in early Homo. In the Homo lineage, body size tends to increase and sexual dimorphism appears to decrease through time.”

iii. Tools! When it comes to tools, we have found quite a bit more than just a broken axe-head and a couple of bones lying next to it a few places:

“The Zinjanthropus site (FLK Zinj) is one of the richest Oldowan sites ever excavated, containing well over 2000 stone artifacts and over 3500 fossil animal bone specimens, with over 1000 of the bones identifiable to taxon (unit of zoological classification, such as species, genus, or family) or body part; more than 90 percent of these belong to larger mammals.” And it’s not the only one of its kind: “At one site FJ-1 [Fejej, Ethiopia], almost 2000 artifacts have been excavated from two major layers”. The Nyabusosi (Site NY 18) site in Uganda, “dated to 1.5 million years ago, contains an assemblage of approximately 600 artifacts”. Excavations at Senga 5A in DR Congo “have yielded hundreds of artifacts, along with many fossil animal bones.”

More generally,

“The earliest archaeological sites found have been assigned to the Oldowan Industry, a term coined by Louis Leakey (1903-1972) and Mary Leakey (1913-1996), based on their work at Olduvai Gorge in Tanzania […] The Oldowan is characterized by simple core forms (the parent piece of rock from which flakes are detached), created from river-worn cobbles and angular blocks of stone; the sharp-edged, angular flakes and fragments detached from such cores (debitage); often battered hammerstones; and occasional retouched pieces (usually flakes, the edges of which were further modified by striking off tiny chips to reshape or sharpen the edge) […] Most archaeologists now group the Oldowan and Developed Oldowan into the “Oldowan industrial complex” (Isaac 1976)” – yeah, you got that right, ‘industrial complex’. These were toolmaking complexes.

So who made the tools? This is still a bit uncertain, but at the moment Australopithecus garhi is high on the list when it comes to the early Oldowan tools. The estimated cranial capacity of A. garhi is just 450 cc, i.e. comparable to gorillas and chimpanzees. “Many paleoanthropologists consider this species to be a probable maker of early Oldowan tools and a potential ancestor for the genus Homo.”


September 11, 2011 - Posted by | Anthropology, Archaeology, Evolutionary biology

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