BOTTLENECK

Two Gardens of Eden

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Homo antecessor had separated from our direct ancestors about 1.5 million years ago.  At that time they had moved west through Europe by following animals that had come from Africa and/or Asia.  Their ancestors had most likely lived in Ukraine or thereabouts before they had followed the animals west, but after  had separated from them, or at the same time, this population split into multiple parts, and two of the parts moved south:  One part of this population moved south into the swamps of southern Mesopotamia while another part moved into Africa.  We will label those who moved into southern Mesopotamia Group A and those who moved into Africa we will label Group B.

Those in Group A barely survived.  Their total effective population was reduced to about 1,200 individuals, and their numbers remained low for about 100,000 years.  This type of drastic reduction of a total effective population is known as a population bottleneck.

Following the population bottleneck, after their population had rebounded, some part of the population moved north into Europe.  These were the ancestors of Homo neanderthalensis.  The earliest fossils of Homo neanderthalensis have been found in a cave in the Atapuerca Mountains of northern Spain and dated to around 425,000 years ago, and there is other evidence of their early presence in southern Europe, including at a site on the Italian Peninsula where a huge elephant was butchered.  The butchery site was dated to about 400,000 years ago.

Subsequently, there were two more splits, within 60,000 years of the movement of early into southern Europe:  One group, a southern branch of the Homo denisova clade, moved eastward through South Asia and Southeast Asia, leaving some parts of their population along the way -- in South Asia and elsewhere along the way to Southeast Asia.  Then a rise in sea levels left some of those in Southeast Asia stranded in what is now Island Southeast Asia, offshore from mainland Southeast Asia.

This early migration constituted a southern branch of the Homo denisova clade, but then this southern branch split, as some moved north to constitute a northern branch which moved north into the foothills of the Tibetan Plateau, where they started migrating seasonally between lower and higher elevations.  During warmer periods some of them left their fossils on the Tibetan Plateau itself.  Those who spent time on the Tibetan Plateau evolved special adaptations that enabled them to remain active at high altitudes.

These migrating Homo denisova were ancestral to  others whose remains have been identified as belonging to them, based on physical features of their bones, DNA recovered from bones and teeth, and proteomic analysis.  These remains have been found in various parts of East Asia as well as on the Tibetan Plateau and in the Altai Mountains (at Denisova Cave).

While some members of Group A had moved north and west into Europe and given rise to Homo neanderthalensis and others had moved east and given rise to Homo denisova, still others had remained in southern Mesopotamia.  These were the ancestors of archaic humans who recombined with Group B, most likely in Africa, and then gave rise to our African ancestors.

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 recombined with Homo neanderthalensis

 Fossils of Homo neanderthalensis have been found in a cave in the Atapuerca Mountains of northern Spain and dated to about 425,000 years ago.  It isn't known if the DNA extracted from these early  included DNA contributed by later migrants from the Levant.

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 the 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 to make hand-held cutting tools 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 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 animals that were less dangerous than elephants; under most circumstances they avoided healthy elephants.  Their favorite were steppe bison, which were brave but stupid.  Aurochs were sometimes hunted, but they were also 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, but he also knew that he was expected to be brave.  And he knew little about 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 describe to less experienced hunters how they should approach various kinds of animals and what to expect from them.

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 wondered what the purpose was of collecting the heads of horned animals.

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, 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.​