HOMO IN AFRICA

Onset of the Pleistocene

Kenyanthropus platyops were unable to survive the climatic changes that occurred in the Rift Valley of Africa prior to the glacial maximum that marked the began the Pleistocene epoch about 2.58 million years ago, and the changing climate forced Australopithecines to rely more on tough, fibrous foods, so that they evolved gigantic jaws and jaw muscles.  But even more drastic climatic changes occurred in Europe.  Our own ancestors did survive that first great glacial maximum, by moving into Africa.  This is shown by the early Homo fossil found at the Ledi-Geraru site in the Afar region of Ethiopia and dated at about 2.8 million years ago.

The Homo fossils found at the Ledi Geraru site include only a jawbone and teeth, so the species can't be named, but the fossils definitely belonged to a member of the Homo clade.  Like other members of the Homo clade, the Ledi Geraru Homo most likely had evolved in Eurasia prior to moving into Africa.

Further south along the Rift Valley, in Kenya, archaeologists have found the tools that were made and used by these early Hono,  These tools included sharp flakes and hammerstones, and archaeologists also found rock cores from which the flakes had been expertly struck.  More than a thousand of these stone artifacts have been found in northern Kenya, showing that these earliest African Homo consistently made these tools over a period of 300,000 years, during a time that preceded their increase in brain sizes.  This indicates that increased brain size was not needed for the production of such tools.  Thus, Homo fashioned stone cutting and pounding stones because their way of life required them to do so and the Australopithecines did not make them because they did not need to.

Behaviors such as stone tool production are based on teaching and learning behaviors, not on inventiveness.  Teaching and learning behaviors emerged in semi-aquatic hominins, and indeed the earliest known flaking of stone chips from a rock core was done by Kenyanthropus platyops, who were semi-aquatic.  (Their semi-aquatic mode of life is revealed by their cross-stepping gait when hurrying across open country in an emergency.)

Later African Homo

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The earliest members of the Homo clade moved into Africa just as the first great glaciation of the Ice Ages approached.  One of those early Homo species, known in current literature as Australopithecus sediba, returned to arboreal habits and re-evolved many traits of Australopithecus, but their teeth and hands give them away as truly members of the Homo clade.

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Australopithecus sediba

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Australopithecus sediba, when they came to Africa, found trees there that bore trees throughout the year, whereas such trees were unknown in Eurasia, and they re-evolved characteristics that suited them for climbing trees.  Moreover, their Eurasian ancestors had often needed to travel fast cross-country, but Australopithecus sediba seldom needed to do that.  Consequently, they evolved a twisted ankle which was not so good for walking cross-country but helped them to walk on tree branches while they wrapped their flexible feet around branches and grabbed smaller side branches with their big toe. Their hands, even more than  their teeth, show that they actually belong in the Homo clade.

Australopithecus sediba and Homo habilis both had very human-like hands, but the two species used their hands differently​.  Even though Australopithecus sediba did not have long, ape-like hands that were suitable for supporting their weight while hanging from branches, they often did partially support their weight by holding onto branches.  The resulting strain on their finger bones caused the phalanges of the forefinger and two other fingers to become thickened by the strain that was imposed on them.  Australopithecus sediba also evolved especially long and powerful thumbs to partially make up for their shorter hands, and they had re-evolved a slight curvature of their finger bones that is similar to what is observed in the hands of Australopithecines.

Homo habilis

Homo habilis had straighter fingers than Australopithecus sediba, and their hand bones show evidence of loading throughout their lifetimes that was very similar to that of modern humans, who seldom use their hands for climbing trees.

Homo habilis relied more on what they could salvage from animal carcasses, rather than climbing trees for fruits, but Homo habilis still had to climb trees on occasion in order to escape from predators or to forage in trees, and they still had shorter legs and longer arms than some later members of the Homo clade.  Their hands, however, were well suited for grasping a rock firmly so as to strike it accurately during rock-chipping operations.  A firm grip was also needed for throwing rocks accurately, so as to deter predators or drive them away from carcasses.

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Although they were generally able to defend themselves against medium-size carnivores, they were not good at stealing carcasses from them, so Homo habilis often had to make do with scraps of skin and bones that were not eaten by carnivores.  They could obtain fresh meat by killing herbivores, but they were probably less successful at it than later members of the Homo clade.

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Some archaeologists have posited that Australopithecus sediba were ancestral to Homo habilis, but it was more likely the other way around, or the ancestors of Australopithecus sediba might have just happened to inhabit an area where fruiting trees were abundant, and adapted to those conditions, whereas the ancestors of Homo habilis retained the characteristics of their ancestors.

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Homo rudolfensis

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Other hominins, Homo rudolfensis, were similar to Homo habilis.  They also had human-like hands and most likely used them for similar purposes, but they had even larger molars with flat chewing surfaces, meaning that their teeth were well suited for efficiently chewing tough fibrous plant materials.  Yet they did not have the huge jaws and huge jaw musculature that enabled the Paranthropines to chew on their food all day, extracting nutrition from very low-quality foods.

Why not?  The answer is that Homo rudolfensis used their enhanced mental capabilities to find higher quality plant materials that hid underground, like roots, bulbs and corms, and to dig them out of the ground with sharpened digging sticks and stone tools.  Moreover, they most likely pounded the tough plant materials and cut them into pieces to make them easier to chew.

Still, Homo rudolfensis as well as the later Homo habilis were a dead-end street of evolution.  They became extinct when the African climate continued to dry and the foods they relied on became unavailable in the places where they lived.  Later members of the Homo clade were better able to cope with the changing climate of Africa, and further evolution of the Homo clade involved evolution of more resilient feet, longer legs and shorter arms, all of which helped their descendants to travel long distances.  Their evolution also involved adaptation of their hands and shoulders that made it easier for their descendants to make and use tools and weapons.  Their brain sizes also increased as controls on proliferation of neurons were loosened.  Surprisingly, molar sizes decreased steadily over the course of the next two million years, even though the early members of the Homo clade often relied on the uncooked starches that they obtained from eating the underground parts of plants.

The digestive tracts of hominins followed a similar trajectory, reducing in size as hominins found higher-quality foods even in vegetative matter and became better at hand-processing their foods.

All of the above

The human-like hand construction of the above early Homo leads us to believe that they were all descendants of veteran stone-chip makers, and the lifestyles of all but Australopithecus sediba lead us to presume that they continued to make crude stone chips until they drifted into extinction.  They were doomed to extinction because they lingered in eastern Africa where the climate was often so dry that grazing and browsing animals became rare, and the hominins there were forced to rely almost entirely on very low-quality foods.  Our own ancestors, however, retreated to Eurasia and/or to southern Africa.

Bone choppers

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These early Africans (one or all of them) invented a new type of stone tool:  bone choppers.  The bones of freshly killed animals presented a special problem for Homo habilis and other members of the Homo clade because the bones were not brittle, like those that had had time to dry out, and when a hominin hit a fresh bone with a hammerstone it would splinter badly.  And the tiny splinters that were left in the marrow were not good for the stomach.  Homo habilis or some other early member of the Homo clade responded to this problem by inventing bone choppers.

The first bone choppers were probably created by accident, when an early Homo removed chips from one edge of a rock.  As he did this the edge would be whittled down until it was sharp.  Then, if this sharp edge was struck against a large bone so as to put a dent in it, the bone could then be broken at that point without splintering badly.

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Later hominins who were more closely related to ourselves than Homo habilis either copied the design or reinvented bone choppers, and then went on to invent hand axes, which were good for efficiently butchering any large or medium-sized animal.