AFRICAN HOMININS

Desiccation of the Mediterranean Sea

During the seven hundred thousand years that followed, after the Trachelos Creature left those mysterious footprints on a peninsula that extended from the northern shore of the Mediterranean Sea, the inland sea fluctuated wildly in its water level, because it wasoften disconnected from the Atlantic Ocean at Gibraltar, and water inflow from rivers was insufficient to keep it filled.  During those seven hundred thousand years it was reduced several times to saline lakes that could not support the ocean life on which semi-aquatic hominins depended for sustenance, and our ancestors were forced to find sustenance in swamps and other shallow waters, where their own ancestors had lived.
They sometimes wandered across the landscape, looking for the remains of terrestrial animals whose carcasses they could claim and scavenge.  The best and most nutritious food that was available to them during this time was bone marrow from the carcasses of very large animals, such as the huge elephant-like creatures that shared the Earth with them.  Traveling across open territory was very dangerous for them, and this often forced them to use sticks to defend themselves, and to learn how to avoid and fend off predators that tried to attack them.

Earliest Africans

Even prior to the salinity crisis, in which the Mediterranean Sea turned into saline lakes, some apes and hominins were moving south, into Africa.  There, they found trees that bore edible fruit throughout the year.  The earliest of thes new Africans were Sahelanthropus tchadensis, who left their fossils near the southern shore of the Mediterranean Sea, and who were more likely ancestral to modern gorillas or to chimpanzees than to hominins.  Orrorin tungenensis (who might better be classified as hominins) followed on their heels, and left fossil remains in what is now the Tungen Hills of Kenya.

The salinity crisis prompted many descendants of semi-aquatic hominins to move into Africa:  Ardipithecus kadaba left their fossils in what is now the Afar region of Ethiopis, and these homininns were followed by Ardipithecus ramidus and then by the Australopithecines and Paranthropines.
Ardipithecus ramidus had a very big toe that pointed to the side, like an overgrown thumb; it enabled them to climb trees more easily.  The later hominins, however, all had forward-pointing big toes, like us.

Eurasian hominins

These early African hominins were not our ancestors.  Our own ancestors lived in Eurasia at that time, where few trees fruited throughout the year.  After the Mediterranean Sea dried up, and turned into saline lakes, our ancestors moved inland, into Eurasia, where their ancestors had lived.  There, they lived by killing animals by driving them into deep water and drowning them.  But hominins are not natural carnivores, so they had to supplement their diets with other sources of food energy, such as marrow that they extracted from the bones of large animals whose carcasses had been abandoned by other scavengers.  (We, ourselves, are hominins, and if we try to obtain more than about 50% of our energy from lean meat, we are in danger of being poisoned by a buildup of chemicals left over from incomplete metabolism of proteins.  Natural carnivores, however, can safely obtain 100% of their energy from lean meat, if necessary.)
These Eurasian ancestors of ours wandered about in small groups whose members were highly dependent on each other; they helped new mothers by carrying and holding their infants while the group was wandering about looking for unbroken bones or while the mother was foraging.  This was because the isolated groups of wandering hominins were highly inbred, and if members were lost to predation and reduced in numbers, isolation of an individual group in the vast wilderness usually meant that individual group members did not pass on their genes.  In other words, the Eurasian hominins needed to maintain a minimum group size, or they would be driven into extinction.
But the inbred quality of the groups meant that even if an individual member died in defense of the group, his genes would in general be passed on by other members of the group.  In other words, survival of the genes carried by an individual member of a group of Eurasian hominins depended on the survival of other members of the group to which he belonged.
Group sizes also needed to be maintained in order tp conserve knowledge and skills possessed by individual members of the groups.  In other words, survival of the groups of hominins depended on the knowledge and skills of their members.

Later African hominins

Hominins who moved into Africa supplemented their diets alternative foods that they found there, and they depended less on what they could scavenge from the carcasses of large animals.  This meant that the groups of hominins no longer needed to move fast and for long distances, so as to outdistance other groups of scavenging hominins and find unbroken bones. Thus, the African groups became less isolated from each other, and it became easier for both males and females to move from one group to another.  Thus, the groups became less inbred, and group cohesiveness was no longer so important.

A difficulty that was encountered by these fully terrestrial hominins was that they, like their Eurasian contemporaries, were vulnerable to predation as they walked over relatively open, treeless ground in search of food.  This meant that they had to walk fast; they could no longer afford to waddle, as their semi-aquatic ancestors had done, they didn’t even have time to swivel their hips as they walked as semi-aquatic hominins had done when they were in a hurry.  The solution was that their hips needed to become narrower, so that their center of gravity moved in a straight line as they walked.

Narrowed hips

The problem with narrower hips was that in order to reduce the size of the pelvic girdle, the opening for the birth canal needed to become smaller.  Moreover, the consistent upright posture of the hominins meant that the pelvic girdle was rotated, so that during the birth process the infant needed to pass down through this hole before exiting to the front of the mother’s body.  Now, as the pelvic girdle shrank in size the birthing process became even more difficult because the infant had a harder time passing through the hole in the pelvic girdle before turning and exiting from the mother’s body.
Inn order to accomplish this feat, the infant needed to be passed head-first through the birth canal, and the head needed to be smaller.  The solution was that infants began to be born prematurely, before the brain was fully developed.  This meant that newborn infants were quite helpless, and were unable to forage for themselves.  This is why group sizes became so small among African hominins – so that a male could provide more help for his mate during the critical period while the infant was so helpless.  Eurasian hominins, on the other hand, needed to maintain larger group sizes, and this required radical changes in social arrangements.  We will consider that problem in the next chapter, on Eurasian hominins.

Australopithecines

The Australopithecines were most likely descended from hominins who had lived along the northern shore of the Mediterranean Sea until this inland sea dried up and turned into a series of lakes that were separated by land bridges.  The furthest south of the Australopithecines were the 

Australopithecus africanus.

Fossils of Australopithecus africanus were found in southern Africa.  They had protruding jaws, like apes, probably because these hominins had relied on some of the same foods as apes.
Fossils of the other Australopithecines were found in the Rift Valley, in eastern Africa.  The earliest of these were  Australopithecus anamensis, who lived more than four million years ago and had more "primitive-looking" features.  The other Australopithecines were not necessarily descended from them, however, and most likely evolved separately. Australopithecus anamensis had heavy irregularities in their skulls that weighted their skulls; these heavy skulls may have helped their semi-aquatic ancestors to dive into salt water.

A later-appearing hominin, Australopithecus afarensis, like Australopithecus anamensis,  probably evolved from semi-aquatic hominins who lived along the northern shore of the Mediterranean Sea before it shrank into a series of lakes that were separated by land bridges.  These land bridges allowed hominins to cross into Africa.

The feet of Australopithecus afarensis, like ours, were not well suited for climbing in trees, but not really well suited for walking on dry land, either.  They were better suited for wading in water; and they were much more flexible than our own, but maybe not flexible enough to wrap around the branch of a tree.  Their toes were long, able to grasp a hanging vine, but they all pointed forward, making it easier for them to walk.  An infant of the species could use his long toes to grasp the hair on his mother's back, which he might have to do if she received insufficient help from her mate.  (Australopithecines may have regained some of their body hair when they moved into Africa.)

Members of this species had shoulder joints that pointed up, rather than down, indicating that climbing trees was still important to them.  Moreover, their arms were rather long as compared with ours, and their legs rather short, making it somewhat easier for them to clamber up trees.  Their hands were long and their fingers slightly curved, making it easy for them to grasp a limb of a tree.  Their thumbs were short, and not as well constructed as our own for firmly grasping an object such as a rock.

Some part of the Australopithecus afarensis population seems to have separated from the others and started to specialize in hard foods, such as crabs or hard nuts.  Consequently, they evolved to have stronger jaws and thicker enamel on their teeth, and are often referred to as Australopithecus deyremeda, but some archaeologists consider them to be a subspecies of Australopithecus afarensis.

Some of the Australopithecines moved further south, into what is now South Africa, where they evolved separately from Australopithecus afarensis.  They had protruding jaws that gave them a much less "human" look, and archaeologists called them Australopithecus africanus.
Australopithecus garhi, whose fossils have been found in the rift Valley of Africa and dated at about 2.5 million years ago, were most likely descendants of Australopithecus afarensis or of some other Australopithecine and differ markedly from hominins who came into Africa more recently.   Australopithecus garhi had evolved huge jaws and huge jaw muscles that enabled them to spend a lot of time chewing on tough, fibrous foods.  The increasingly dry climate of Africa must have forced them to make do with such foods on occasion.  (Africa, especially eastern Africa, became increasingly dry in eastern Africa between three million years ago, when Australopithecus afarensis disappear from the fossil record in the Rift Valley, and 2.5 million years ago.  Later on, some fossils of Australopithecus garhi appear there.)

Australopithecus garhi had arms that were long and strong, like those of Australopithecus afarensis, and both species had slightly curved fingers.  These characteristics would help them to grasp tree limbs firmly and climb trees.  But unlike Australopithecus afarensis and the other australopithecines, Australopithecus garhi had long legs like members of the Homo clade.  This indicates that they sometimes had to travel long distances in order to find or catch their food.
Their stone tools have not been found, but fossilized bones of animals that they had butchered were found in association with their own fossilized remains, so they must have made and used stone tools.

Paranthropines

As the climate of Africa continued to dry, some of the australopithecines of Africa evolved huge jaws, huge molars, and huge jaw muscles, and habitually chewed low-quality foods throughout the day.  Archaeologists who assigned Paranthropines their own genus name probably made a mistake, because those whose fossils were found in eastern Africa most likely evolved independently of those of southern Africa.

The southern and eastern Paranthropines were similar in their lifestyles, in that the males collected harems.  The patriarch would eject young males from the harem before they could reach full maturity, and the young males would then wander about in the wilderness, issuing a mating call that attracted predators.

Paranthropines in the two regions subsisted on somewhat different foods, because their environments were different.  Paranthropus boisei, in eastern Africa, subsisted almost exclusively on bulbotubers that they found growing in marshes, whereas Paranthropus robustus, in southern Africa, were omnivorous.

Kenyanthropus platyops

Other African hominins that differed from the Australopithecines were named Kenyanthropus platyops by archaeologists who found their fossils; their small jaws and teeth gave their faces a flat appearance.  ("Platyops" is the word for "flat-appearing" in the classical Greek language,)  These other hominins, unlike Ardipithecus ramidus and the Paranthropines, were semi-aquatic.
Fossils of Kenyanthropus platyops were found in Kenya and dated at about 3.5 million years ago, and their small jaws and teeth indicated that they ate only very high-quality foods that didn’t require a lot of chewing -- like crabs and other small things that lived in the water of shallow lakes.
Kenyanthropus platyops were most likely the creatures who butchered an animal in the Rift Valley of Africa about 3.4 million years ago.  They used sharp chips of stone that they had expertly struck off of a larger rock -- the small stone chips that they had struck off and the larger stone core were found in the same vicinity and dated at about 3.3 million years ago.

Unlike fully terrestrial hominins, who on occasion struck flakes of stone from a stationary rock that they held to the ground, these semi-aquatic hominins struck the small and large stones together by carrying the heavier stone upward in one hand while striking down against it with a smaller stone that they held in their other hand.  This might have been because their ancestors had seldom needed to throw rocks at predators, but often had to paddle long distances.

Throwing a stone required our own terrestrial ancestors to develop great strength in muscles that propelled the throwing arm forward, but this did not require very much strength in muscles that moved the arm in the opposite direction; semi-aquatic hominins, on the other hand, used their arms for paddling in water, which required strong muscles that they used for lifting their arms.Footprints left by  Kenyanthropus platyops were preserved (like those of the Trachilos creature) in muddy volcanic ash that subsequently hardened into rock.  Those footprints indicate that their feet were quite different from those of Australopithecus afarensis, and from our own − they were much shorter and wider and much more flexible, and their foot bones were differently proportioned.  For instance, they had a big toe that stuck out slightly to the side.  These differences indicate that their ancestors must have separated from those of Australopithecus afarensis millions of years before the footprints of the two species were laid down in the Rift Valley of Africa.  Kenyanthropus platyops had remained predominantly semi-aquatic. and their flat faces do not indicate a close relationship to ourselves; they had flat-appearing faces because they ate very high-quality foods that didn't require much chewing and because (like many semi-aquatic animals) they had small, streamlined sinuses in their head rather than large cavities to warm and moisten the air that they breathed.

We can also conclude that the practice among early hominins of deliberately killing terrestrial animals for food must have been ancient, indeed; it must have been passed down by the last common ancestors of Kenyanthropus platyops and our own ancestors of that time.  Of course, members of the Homo clade also made stone tools and used them to butcher animals.  But semiaquatic hominins, in particular, needed to do this because, like ourselves, they were not natural carnivores, and could not tolerate a diet that was almost entirely protein.  (Food that Kenyanthropus platyops found in shallow lakes or that other semi-aquatic hominins found in the intertidal zone were almost pure protein.  Not being natural carnivores, they needed to supplement this diet with fat from terrestrial animals.)

We often think of the Homo as occupying a different environmental niche than the semi-aquatic hominins from which we sprang, but do they, really?  A modern human can do just about anything that Kenyanthropus platyops could do, like walk cross-country (in groups) with little to fear from predators.  The genus, Homo, who were soon to appear on the world stage, were not quite modern humans, but they, also, could compete directly with semi-aquatic hominins on their own turf, and then survive environmental changes that semi-aquatic hominins could not survive.
For instance, Homo could ambush and kill animals by trapping them against a cliff, and they could walk or run cross-country to find their prey.  When the environment of eastern Africa changed, and the shallow lakes dried up, Kenyanthroopus platyops became extinct.

Another peculiarity of these semi-aquatic hominins was their gait, as evidenced by the footprints that one of them left in mud that hardened into stone, is that they cross-stepped, at least while hurrying across open country -- throwing the left foot in front of the right, and throwing the right foot in front of the left -- and they were very good at swaggering in this cross-stepping manner because they had an extra lumbar vertebra -- they had six instead of the five that we ourselves have.
Their swaggering gait was necessitated by their very wide pelvic girdle.  They retained the wide pelvic girdle of quadrupeds, because this left room for a relatively wide opening where the birth canal passes through it.  This wide opening made it easier for them to give birth, because it left more room for the head of an infant to exit from its mother's womb.

Kenyanthropus platyops did not need to be efficient walkers on dry land, because their escape from terrestrial predators was normally into deep water; but when descendants of semi-aquatic hominins became fully terrestrial, they often needed to walk quickly, or even to run.  The Australopithecines, however, who were fully terrestrial, had evolved to have narrower hips, and this made it more difficult for them to give birth.

Interestingly, modern humans have not entirely given up the semi-aquatic niche of our ancestors:  We can swim and dive about as well as they could, and our bodies are almost hairless.