BOTTLENECK

Two Gardens of Eden

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As already noted, about 1.4 million years ago our direct ancestors separated into two groups, Group A and Group B, which moved into southern Mesopotamia and Africa, respectively.  Subsequently, the total effective population of Group A was reduced to a relatively small number, maybe only about 1,200 individuals, and their effective population stayed low for about 100,000 years before rebounding.  This type of drastic reduction of a total effective population is known as a population bottleneck.

Hippopotami

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The hominins in Group A of our ancestors invented techniques for slaughtering hippopotami, which were the largest animals living in that region.  Hippopotami foraged at night and rested in the water during the day, so the hominins needed to creep up on the animals while they rested in the water and wait for two males to fight over a female.  Then they would perhaps rush in and attack one of the fighting hippos and cripple it.

A single hippopotamus could feed a lot of people, and this was fortunate, because it would take a lot of people to track down the animals.  Tracking the animals was likely accomplished by using percussive signals to communicate over long distances.  The percussive signals eventually came to be a lingua franca that enabled hunting groups to communicate with each other even when they spoke mutually unintelligible languages.

Elephants

Meanwhile. Group B found both hippopotami and elephants in Africa, as well as giant bovines.  Spreading into different parts of Africa, members of the Group B population hunted all of them.  But meanwhile, climatic conditions continued to change, and some elephants from Africa moved north into the Jordan Valley, followed by some part of the Group B population.  Consequently, there was a reunion in the Levant between part of Group A and part of Group B.

Archaeologists found evidence of a campsite near a pathway that had been used by the elephants as they had migrated along the Jordan Valley.  This archaeological site is known as Gesher Benot Ya'akov, and among the evidence that was found at the site were the remains of a huge carp fish that was roasted there about 780,000 years ago.  We don't know exactly who roasted the fish, but it could have been members of the combined population that resulted from this reunion.

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Between then and 400,000 years ago, by which time elephants had become extinct in the Levant, many of these Levantine hominins had migrated into Europe, where they later evolved into Homo neanderthalensis, a northern cousin of our own species, Homo sapiens.  Fossils of these early Homo neanderthalensis have been found in a cave in the Atapuerca Mountains of northern Spain and dated to about 425,000 years ago.

Some of the migrants from the Levant. as they moved into Eurasia, turned east instead of west and specialized in following large deer as they seasonally migrated to and from the Tibetan Plateau.  These became ancestors of Homo denisova.

The roasted carp

It isn't likely that the carp fish was roasted because people wanted their food cooked, like modern people often do, but people of that time often roasted their food after it had begun to spoil. They also would sometimes preserve their meat by smoking it, but that wasn't done in the case of the carp, maybe because it didn't provide them with ligaments and long bones from which to hang strips of its flesh, as did hippopotami and elephants.

It was only much later, about 400,000 years or so ago, that most people began to routinely cook their food.  As with most practices of such ancient peoples (toolmaking, etc.) it is hard to say why they were so conservative, but I would guess that it was because such ancient people lived in small groups that were isolated from outside influences.  In order to keep skills and knowledge intact in their small, isolated groups every action such as toolmaking or hunting was treated as a ceremony that had to be performed exactly as specified by custom.  Any deviation from standard procedure could spoil the "magic" that ensured survival.  We can see echoes of the ancient conservatism in modern-day superstition and religion, as well as in obsessive-compulsive behavior.

Of course, these behaviors still persist in modern populations.  In part, this is because modern populations are forced to live cheek-by-jowl with people who have different traditions and different ways of acting, and this encourages people to experiment with different ways to behave or to do things, but the drastically different modern behaviors are also rooted in a modern wave of self-domestication, which tends to promote immature behaviors to persist into adulthood.

Still, these ancient behaviors can be discerned in human behavior even today.  for example, when a pork taboo was inserted into Leviticus only because pork was seldom consumed in the Middle East at that time.  This conservative behavior is an instance of obsessive-compulsive behavior on the level of cultural dynamics, as are the other Judaic and Islamic dietary restrictions and many other religious prohibitions and requirements.  (Since these behaviors no longer contribute toward survival of the human genome, it is proper to refer them as constituting a disorder.)

The discovery of Homo denisova DNA

The existence of Homo denisova was at first attested to only by DNA and bone fragments that were found in Denisova Cave, in southern Siberia.  Fragments of bone containing their DNA, mixed in with the dirt in the cave, were probably deposited on the cave floor along with the feces of hyenas who had chewed on the bones.

These fragments of bone, and the DNA that they contained, seemed insufficient for naming a species, but then intact hominin fossils were found in various parts of China, including in Longjiang County in northeastern China, where a skull was found and identified as belonging to a new human species.  This new species turned out to be none other than Homo denisova, the species whose DNA was found in Denisova Cave.

Similar DNA has been detected in the genomes of modern people whose ancestors lived in Asia or in Island Southeast Asia, and this provides us with clues as to where Homo denisova lived before they became extinct -- people whose ancestors lived in Oceania (islands offshore of Southeast Asia) have especially large amounts of Homo denisova DNA in their genomes.  Nobody knows for sure, however, where the ancestors of modern people in Island Southeast Asia came into contact with Homo denisova.  But the latter might have entered Oceania earlier, when many of the islands were connected to the mainland.​

This likelihood is supported by the finding of a jawbone off the coast of Taiwan.  Molecular analysis has determined that the jawbone belonged to the same species whose DNA was recovered from Denisova Cave in Siberia, and this shows that the species did indeed inhabit territories that are now in Island Southeast Asia (or at the bottom of the ocean).

Tests indicate that a jawbone found at 10,000 ft on the Tibetan Plateauals belonged Homo denisova, and it is similar to the jawbone that was found in Lonjiang County.  Moreover, fossils attributed to another proposed hominin species in China, Homo juluensis, also share similarities to those attributed to Homo denisova.  I think all of these fossils are from the same species, Homo denisova.

The two Eurasian species, Homo neanderthalensis and Homo denisova, obviously interbred when they came into contact with each other.  We know that because fossils of both species contained DNA sequences that they must have exchanged in that way.  In addition, both species interbred with modern humans, on occasion.​