Anthropology 150

Lecture 6: The Early Members of the Genus Homo

For additional images of fossils, visit the Hall of Human Ancestors at the Smithsonian Institution: http://humanorigins.si.edu/ha/a_tree.html. Click on any species and you’ll see photos of the original fossils. You also visit the e-skeletons project at www.eskeletons.org to see virtual fossils.

Background Sometime soon after 2.5 mya, a population of gracile Australopithecines developed a new

suite of derived traits so extensive that they are identified not only as a new species, but a new genus. This is the genus Homo, to which modern humans belong.

The earliest identified species in the genus is Homo habilis, whose fossils are found in contexts dating from 2.4 to 1.8 mya ago.

Homo habilis (2.4 - 1.8 mya) The suite of derived traits for Homo habilis include a larger brain (Cranial capacity 580 -

750 cc.), larger body size and more efficient bipedal locomotion. The brain isn’t just larger, it is reorganized

Frontal lobe reorganizaion: Frontal lobe is linked to planning and concentration on specific tasks. Two areas of the frontal lobe are enlarged:

Broca’s area – part of frontal lobe linked to speech and control of motor activities. Wernicke’s area – part of the parietal lobe linked to language comprehension and also

for the integration of visual, auditory and tactile information.

Homo habilis and Homo rudolfensis Early Homo was diverse and widespread throughout east Africa. As is normally the case

in such an adaptive radiation, significant diversity developed between the populations. There is disagreements among how to classify these diverse fossils. Some scholars propose splitting Homo habilis into two species: Homo habilis and Homo rudolfensis. The difference between the two populations is clear: Homo rudolfensis is larger, with greater overall stature and a larger brain. The question is whether the differences actually reflect speciation, or whether the differences merely reflect normal inter-species variation. For our purposes, we can talk about all these fossils as members of a single species, albeit with differences between populations.

Differences between Homo and the last Australopitchecines The gracile Australopithecines develop into Homo via anagenesis, but the robust

Australopithecines survive and become even more robust (Australopithecus boisei). To recap, the robust Australopithecines develop very specialized adaptation linked to ingestion of heavy vegetable matter. They stayed small brained.

Homo habilis evolves out of the gracile australopithecine line, with an omnivorous diet.

The species develops a larger brain, becomes cultural.

Homo habilis and the robust Australopithecines co-exist for roughly 1.6 million years. There is some evidence for interaction: their bones found at same sites on occasion, but context is unclear. Some think Homo ate Australopithecus boisei. If so, it was through scavenging, not hunting.

Tool use and Homo habilis As mentioned in the last lecture, early stone tools may be associated with

Australopithecus boisei, but the evidence is circumstantial. With Homo habilis, however, the evidence is undisputed.

Making and using stone tools was a very important adaptation. Australopithecines may have had tools, but they were rudimentary. The earliest stone tools are called Oldowan. They are very rudimentary: simply sharpened stones. You take one rock, hit it with another rock and remove a few flakes from the margin, thereby forming a sharp edge.

Dispute their rudimentary nature, Oldowan tools were a very important adaptation. The opened up a new ecological niche the included the consumption of meat.

Homo habilis was not a hunter, it was a scavenger. It fed on carcasses that had been abandoned by the primary predators. Stone tools were used as part of scavenging, especially to crack bones in order to extract the marrow. Here is the fundamentally new niche: there are very few animals that can crack the heavy marrow-bearing bones of a large animal like a caribou, but those bones contain a treasure of calories rich in protein and fat. By some estimates, the marrow in a single caribou femur may contain up to 2000 kcal. That is enough calories to sustain a modern person for day.

The primary predators (big cats) who were contemporary with H. Habilis could crack those bones. Cats gnaw on the ends of bones, but they don’t crack them. Hyenas were the only competition for that resource. Being able to split bones and get at the marrow was a big deal. Those 2000 kcal can make the difference between living and dying on the east African savannah.

As Sherwood Washburn noted long ago, there was a synergism between tool use, meat consumption, and the expansion of the brain. Brains need lots of calories. It’s estimated that 20% of a modern human’s daily calories go to support brain functions. The brain also requires a lot of amino acids, which come from proteins, which are contained in meat. Brains are expensive and in an evolutionary sense they aren’t worthwhile unless they help you get more food above and beyond what is needed to support them. Using tools let Homo habilis get those extra calories by eating the dead stuff left by the primary predators. More importantly, the calories of those scavenged carcasses were rich in proteins.

The origins of cultural behavior The importance of Homo habilis’s behavior is that it is clearly cultural. Tool use is

cultural: it is learned, shared, and transmitted. Using tools had to be learned because it isn’t part of hominid’s genetic makeup. It clearly

was shared, because Oldowan tools are found all over east Africa. The knowledge of how to make stone tools was also transmitted: Oldowan tools were made for over 700,000 years, from roughly 2.5 mya to 1.8 mya.

The brain of Homo habilis shows an expansion of areas that as especially important for culture. Broca’s area is linked to speech and motor activities. Wernicke’s area is also liked to language as well as the integration of abstract information. No one thinks that Homo habilis had language abilities equivalent to modern humans, but it probably had some kind of rudimentary ability to communicate. More importantly, it had the ability to think abstractly and to think symbolically. Those things are essential to learning, sharing and transmitting culture.

Homo habilis is the watershed in hominid evolution, where culture becomes an important means of adaptation. Cultural behavior is, by definition, non-instinctual; it is learned behavior rather than biologically programmed. Ultimately, however, the ability to be cultural is based on biology and physiology. The ability to be cultural was the product of natural selection: cultural behaviors made the difference between living and dying for hominids.

Homo habilis, the cultural hominid, was extremely successful. It radiated and speciated. Eventually it gave rise to a new species: Homo erectus. The robust Australopithecines were not cultural. Their adapation was narrow and exclusively biological. When environmental conditions changed, Australopithecus could not adapt and went extinct at about 2.0 mya. Because cultural behavior is much more malleable than biologically based behavior, Homo could adapt more quickly by developing new cultural forms.

The extinction of the robust Australopithecines trimmed the hominid line down to a single genus, Homo. How many populations of Homo existed at 1.8 mya ago is unclear, but undoubtedly not all of those survived, either.

Homo erectus

At about 1.8 mya one population of Homo habilis developed new mutations that lead to a new species of Homo, Homo erectus. Early Homo erectus fossils overlap with the last Homo habilis fossils for several hundred thousand years. This dating is evidence that not all Homo habilis populations developed the mutations that led to H. erectus. Some Homo habilis populations simply went extinct

Physically, Homo erectus. manifests several trends that we’ve discussed in the hominid line. It gets taller, going over 5 ft. tall. There is one specimen, a juvenile male who died at about 12 years old, that could have reached a height of 6ft at maturity.

It’s facial structure is less prognathic, less projecting. H. erectus sagittal keels (not crests). Cranial capacity increases. Homo habilis had a cranial capacity of between 450 and 650 cc. Early homo erectus had an average capacity of 900 cc. Late homo Erectus had a capacity of nearly modern dimensions, 1000 -1200 cc.. The shape of the cranium is often described as a "deflated football” (an American football). It has a Pentagonal shape to the back of the cranium and impressive brow ridges.

Erectus is a very robust species, undoubtedly much stronger than modern humans. The post-cranial skeleton (from the neck down) is very much like that of a modern human, simply more robust because their muscles did a lot more work than ours.

Homo erectus teeth were intermediate between Australopithecines and modern humans in both shape and size, and the teeth (especially the molars) get smaller over time. This trend probably reflects a change in diet to softer foods, including more meat and possibly cooked food.

Homo erectus behavior

Homo erectus made a more sophisticated form of stone tools, called Acheulean. Acheulean tools are much more sophisticated than Oldowan tools: they are symmetrical and have very regular edges, which makes them much more effective for cutting.

The archetypal Acheulean tool is the handaxe. Handaxes are leaf-shaped, and worked all the way around the stone. What is remarkable is that wherever and whenever they are found, handaxes have very standardized proportions of height to width.

The latter aspect indicates a very important difference between Oldowan and Acheleuan technologies: Oldowan tools were very crude and almost random in their shaping; flakes were removed from the stone in a very unformulated manner. Acheulean tools show that their makers knew what they were going to make. That is, before starting work on the stone, a Homo erectus tool maker was able to form a mental image of what the final product would look like and was able to formulate the series of steps needed to produce that final product. That is a much more sophisticated form of behavior than the abilities of Homo habilis would have permitted.

Homo erectus subsistence Whereas Homo habilis was a scavenger and obtained meat from the carcasses of animal

killed by other species, Homo erectus was a hunter. There are a great many sites where Homo erectus kills have been found. One of the most impressive is Olorgesaillie, Kenya. At that site Homo erectus hunted large number of baboons, dispatching them and butchering them on the site with stone tools.

In Spain, the site of Torralba/Ambrona has evidence that Homo erectus hunted very large animals, including mammoths.

What these and other cases indicate are a remarkable degree of group cooperation which ultimately depended upon the ability to communicate. This, in turn, is evidence of the more sophisticated cultural abilities facilitated by an expanding brain.

The Diaspora The very successful cultural adaptations of Homo erectus led to population growth and an

adaptive radiation that took the species out of Africa. The dates of the radiation are quite early By 1.7 mya Homo erectus is found in Central Asia at the site of Dminisi, Georgia. By 1.7-1.6 mya Homo erectus is found in southeast Asia, in Java. By 780,000 years ago, Homo erectus is found in western Europe at the Sima de los

Huesos locality (also known as Atapuerca).

Differing tools The eastern Asian archaeological assemblages associated with Homo erectus lack the

Acheleuan handaxe. Why is this so attracts many different opinions. Here are some possibilties

1) The adapative radiation out of Africa took place prior to the development of the Acheulean tool kit. The early dates for Dminisi, which includes individuals that some scholars believe look more like Homo habilis than Homo erectus, can be cited in support of this position.

2) Homo erectus populations east of India may have used materials other than stone for many purposes. It has been proposed that in east Asia Homo erectus could have used materials

such as bamboo to make cutting tools. This proposition is very speculative, but the fact that the Acheulean handaxe never makes it east of the Indian sub-continent leads many to think that cultural factors are in play here.

The handaxe is found everywhere Homo erectus goes, except east Asia. In all portions of Europe you see it, for example.

Changes Homo erectus was around for a very long time. The first fossils appear ca. 1.8 mya ago,

and the last are dated to about 500,000 years bp (before present). Over this time Homo erectus accumulated significant mutations and, through anagenesis, became increasingly like modern humans. Remember that even the first H. Erectus fossils display what is a very modern post- cranial skeleton (the parts from the neck down). The late Homo erectus cranial capacity (over 1200 cc) is within the modern range. The last Homo erectus had some distinctive cranial traits (the flat topped skull, and large brow ridges, but behaviorally they were essentially modern.

Phylogenetic arguments The adaptive radiation of Homo erectus led to a great deal of differentiation within the

species. Some physical anthropologists see the need to classify the populations of differing regions into separate species categories.

Those who favor splitting Homo erectus propose the following scheme;

The Asian population of Homo erectus retains the name Homo erectus because the species was first defined on the basis of fossils found in Java in 1896. By convention, the first named fossils retain the name given to them.

The African population is given the name Homo ergaster.

The European population has several names, but the term Homo antecessor, used for the Atapuerca fossils has some cache.

We will not use this designations. I’ll simply refer to the different populations as Asian Homo erectus, African Homo erectus, and European Homo erectus. I simply want to emphasize, however, that adaptive radiation always leads to differentiation. Whether those differences deserve the definition of new species is a matter for physical anthropologists to sort out.

Next: Archaic Homo sapiens, Neandertals, and Modern Homo sapiens,