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.

Migrations into western Eurasia, Oceania and eastern Eurasia

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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.

Worldwide climates fluctuated much more rapidly than they had before, and by 404,000 years ago climate along the southern fringe of Europe had become warm and moist, and this had attracted huge elephants, much larger than modern African elephants, to move into southern Europe.  When one of these huge beasts died naturally or was slaughtered it was a bonanza for these early Homo neanderthalensis.

Archaeologists have found the butchered remains of one of these beasts in what is now Italy.  There were no suitable stones in the region for striking off large flakes of stone, so a novel technique was used for cutting the meat away from the bone:  Small chips of stone were inserted into wooden clubs, and the flesh was shredded away from sheaths by pounding the flesh.  This yielded strips that were suitable for eating, smoking or cooking but did not leave characteristic cut marks on the bones.  Instead, the archaeologists only found bruises, fractures and grooves on the bones.

The migration into Eurasia didn't all happen at one time.  Rather, there were periodic movements into Eurasia and simultaneously along a southern route through South Asia and Southeast Asia and into Oceania.

Those who migrated along the southern route were ancestors of another branch of humanity known as Homo denisova, who later contributed some of their DNA to modern Oceanians, whose  ancestors migrated into the region as early as 70,000 years ago.

While the earliest migrants into Eurasia were ancestral to Homo neanderthalensis some later migrants, after moving into Eurasia, turned east rather than west and specialized in following large deer that migrated seasonally to and from the Tibetan Plateau.  The hominins who had turned east became ancestors of another branch of the Homo denisova genome, and they later contributed some of their DNA to modern East Asians.

The bravery of horned animals

Homo neanderthalensis were horned animals, and so were elephants --  men carried artificial horns when they hunted and elephants had horns that grew out of their mouths.  But immature boys carried only toy spears, and they were not allowed to hunt dangerous horned animals.

When a boy came of age his initiation was to accompany the men on a hunting expedition.  He was expected to accompany the men as they crept up to a herd of wild horned animals and then rushed at the animals.

There is a limit to bravery, so humans usually chose to fight with less dangerous animals; under most circumstances they avoided healthy elephants, which were quite dangerous.  Their favorite animals were steppe bison, which were brave but stupid.  Aurochs were sometimes hunted, but they were quite dangerous.  Red deer were hardly worth the trouble.

When the men managed to bring down an animal, it was invariably one of the bigger ones in the herd, mainly because the larger ones were brave, and often turned back to fight with the humans.  This made a boy's first initiation to hunting a terrifying experience.  He knew the animals were dangerous, and he knew that he was expected to be brave.  And he had no idea of how he could protect himself without being a coward.

One day a boy who had seen only one hunt but wished to prove his bravery ran to the side at the last second and was gored by an animal that turned back to challenge them.  Afterward, the men discussed what had gone wrong.  It was decided that the boy had not been carefully enough instructed about the characteristics of the various animals.

One difficulty that was often encountered during such instruction was that the youngsters were often uncertain as to which kind of animal was being discussed.  So, the next time a horned animal was killed its head was removed and placed in an unoccupied cave that was to be used for i for instructional purposes, and periodically an experienced hunter would explain to others how the animal should be approached and how it should not be approached.

In the course of time, a collection of such heads, with the horns attached, accumulated in the cave, and thousands of years later archaeologists who found the collection in the cave wondered what the purpose was of collecting the heads.

XXXIt 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.XXX

XXXIt 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.XXX

XXXOf 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.XXX

XXXStill, 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.)XXX

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.​