HOMO IN AFRICA

Still more African hominins

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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.

The first two million years

XXXIn the next section we will begin our discussion of a two-million-year period that culminated with the emergence of archaic Homo sapiens -- the period from 2.4 million years ago to 400,000 years ago.  About 2.4 million years ago, as the first great glacial period of the Ice Age approached, our ancestral population retreated to the tropics, in Africa.  Then as the planet again warmed, our ancestral population divided into two parts.  One of the two parts, probably the more important of the two, returned to Eurasia while the other part went further south, into southern Africa.XXX

XXXThe southern population was primarily ancestral to the wide-ranging set of related species that is generally subsumed under the heading Homo erectus sensu lato.  This population, radiating mostly from the southern population, spread into parts of Africa, across the southern rim of the Eurasian landmass and into Southeast Asia while the northern population, during warm and wet periods, dispersed into a band that stretched from western Europe to the western foothills of the Tibetan Plateau.XXX

XXXThe two populations followed similar trends with regard to body size, brain size and cognition:  body and brain sizes increased rapidly, especially in the southern population, and humans began a long, slow switch from mostly gestural languages to mostly vocal languages -- a switch that took at least two million years to complete.  The switch to vocal languages was accompanied by increasing mastery of language and by continuing increases in brain sizes, most likely driven by linguistic and other cultural factors.XXX

XXXModern populations appear to be primarily descended from the northern ancestral population that evolved in Eurasia, and this ancestral population eventually separated into two parts.  This bifurcation of the population was somehow associated with a hundred-thousand-year population bottleneck that might in turn have been associated with a mismatch of chromosomes caused by fusion of two chromosomes to form Chromosome 2.XXX

The next three sections of my story delineate the evolution of our ancestors during the period from 2.4 million years ago to 400,000 years ago.  This two-million-year period begins with the earliest members of the Homo clade and ends with archaic Homo sapiens, who were by then clearly on the way toward driving every other hominin into extinction.  This is a pretty big chunk of evolution, so in the next section, on the Homo clade itself, I will cover the period from 2.4 million years ago to 1.4 million years ago.

In the course of this first half of the two-million-year period most species that belonged to the Homo clade became extinct, and we are descendants of a species that inhabited both Eurasia and Africa at the end of that one-million-year period:  Homo heidelbergensis.

In the following section I will begin the discussion with two trends that continued throughout the two million years and did not end until about 50,000 years ago.:  the switch from mostly gestural languages to mostly vocal languages and the constantly increasing scull sizes of our ancestors to accommodate their swiftly evolving brains.