BOTTLENECK

A Garden of Eden

​​

As already noted, about 1.5 million years ago a group our direct ancestors separated from a second group and moved into the vicinity of the swamps of southern Mesopotamia.  I will call these two groups Group A and Group B, respectively, and these two groups constituted the total population of our ancestors at that time.

In the course of the next 600,000 years or so, the total population of Group A seems to have been reduced to a relatively small number, perhaps to as few as 1,000 individuals.  This decline in total effective population may have been caused by the fusion of two chromosomes, to form Chromosome 2.  Such an event could instantly reduce the total effective population, because the mismatch of chromosomes could make it extremely difficult for those with the fused chromosome to produce viable ofspring by mating with individuals without a fused Chromosome 2.

If this is the reason for the decline of the effective total population, it may have taken about 100,000 years to clear the mismatch of chromosomes by the gradual elimination of the population of those without the fused Chromosome 2.  The process by which this occurred is unknown,

​

Hippopotami

​

The hominins in Group A of our ancestors invented techniques for slaughtering hippopotami, which at that time 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 rush in and cripple one of the animals.  The result was often a dead hippo floating on the water, which they would drag to dry land and butcher.

​

A single hippopotamus could feed a lot of people, and this is fortunate, because it took a lot of people to track down the animals.  This was done by utilizing percussive signals to relay information about the movements of the animals.  These percussive signals came to be a lingua franca that enabled hunting groups to co communicate with each other even when they spoke mutually unintelligible languages.

Elephants

​As the population of Group A increased, young people sometimes explored along the banks of the rivers that fed the swamps, and one of these groups stumbled upon migrating elephants.  Exploring further, they found elephants migrating along the Rift Valley to the west.  They gave up the idea of trying to ambush one of the animals, so they returned home unscathed, but soon hunting groups from the swamplands followed in their footsteps and started hunting and slaughtering the animals, which were larger than the elephants that inhabit parts of Africa today.

Archaeologists found evidences of a campsite near a pathway that had been used by elephants as that had migrated along the Rift Valley 800,000 years earlier.  This archaeological site is known as Gesher Benot Ya'akov, and among the evidences that were found at the site are the remains of a huge carp fish that was roasted there about 780,000 years ago.

​

Between then and 400,000 years ago, by which time elephants had become extinct in the Levant, many of these Levantine hominins from Group B 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.

Homo heidelbergensis

Another large-brained hominin species, Homo heidelbergensis, had lived in both Africa and Europe Europe earlier, but had vanished from the Europe long before the ancestors of Homo neanderthalensis arrived there. The African Homo heidelbergensis, however, were still quite numerous, particularly in eastern and northern Africa, so any part of Group A that had moved into Africa around 400,000 years ago or earlier would have been absorbed into the Homo heidelbergensis population.

Homo heidelbergensis is a catch-all species to whom large-brained members of the Homo clade are often assigned if they don't seem to belong anywhere else.  The species was named after a huge jawbone that was found near Heidelberg, Germany.

Homo neanderthalensis and Homo longi/denisova

In any event, hominins from Group A that migrated from the Levant seem to have given rise to two species in Eurasia.  One of these species, as noted, was Homo neanderthalensis, whose more-or-less fully evolved remains were first found in the Neander Valley in Germany.  The other European species which seems to have evolved from Group A was a species whose existence was at first attested to 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 skulll was found and identified as belonging to a new human species.  The DNA that was found in Denisova Cave most likely belonged to the same species, so I will call them Homo longi/denisova.

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 longi/denisova lived before they became extinct.  People who live in Oceania (islands offshore of Southeast Asia) have especially large amounts of Homo longi/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 longi/denisova.  But the latter might have entered Oceania earlier, when the islands were connected to the mainland.​

​​​​

The two Eurasian species, Homo neanderthalensis and Homo longi/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.​

​

Concerning some popular assumptions

Lots of African Homo heidelbergensis fossils have been dated to between 700,000 and 400,000 years ago, but as the magic date of 320,000 years ago approaches (in the archeological record) the fossils become exceedingly rare.  This has allowed some archaeologists to presume that features of the archaic Homo sapiens skull that differentiate the latter species from Homo heidelbergensis -- compressed face, tall cranium and sharp chin -- evolved from the quite different features of African Homo heidelbergensis, and that this evolution occurred right before the archaic Homo sapiens face began to appear in fossils that were found in various parts of Africa.

​

Backing up this theory is the fact that human genetic diversity is higher in Africa than anywhere else on Earth.  However, there is evidence that archaic Homo sapiens did survive in the Middle East until late in the Holocene.  They might have been almost entirely wiped out by Sumerian farmers -- we will deal with that later.

​

New evidence shows that ​African Homo heidelbergensis were actually contemporary with the sudden appearance of archaic Homo sapiens, in various places around Africa:  A Homo heidelbergensis fossil found in Zambia has been redated to around 300,000 years ago.

The archaic Homo sapiens populations that popped up about that time had several distinctive features in common, including high, domed foreheads. flat and compressed faces, and sharp chins.

We might entertain the possibility that Homo sapiens evolved from a subset of the African Homo heidelbergensis, but then we have to explain how this subset had remained separated from the rest of the Homo heidelbergensis population for a million years or more, so I think it is time to accept that the sudden appearance of Homo sapiens features in the African fossil record is indicative of an influx of hominins into Africa from someplace else.  I suggest that they came from the Middle East.​​​​​​​​​​​​​​​​​​​​

​

Incidentally, descendants of the hominins who hunted Elephants to extinction in the Levant seem to have survived there ​until about 130,000 years ago.  Their fossil remains were found at Nesher Ramla in central Israel, and stone tools that they had produced were found nearby.  Both the fossil remains and the tools were dated to roughly 130,000 years ago.

The origins of Homo sapiens.

As I have noted, our direct ancestors split into two groups, which I call Group A and Group B, about 1.5 million years ago and recombined about 300,000 years ago.  We have followed Group A into the swamps of Southern Mesopotamia, and some part of Group A then moved into the Levant.  Then, some of these Levantine contingent of Group A moved back into Eurasia and evolved into Homo neanderthalensis and Homo longi.denisova.  Based on this narrative, we can now identify the other part of our ancestry -- the part which split from Group A 1.5 million years ago and then recombined with them 300,000 years ago.  They were none other than the ancestors of Homo heidelbergensis.

DNA analysis of modern humans indicates that as Group A recombined with Group B, functional DNA quickly disappeared from the hybrid (our own direct ancestors).  So, although about 20% of our own DNA came from Group B, almost none of it is functional!  In other words, Homo heidelbergensis functional DNA was largely incompatible with our inheritance from Group A, and was quickly eliminated from our genome.

Percussive signals

​​

Now that we have solved that riddle, let us return to our narrative about our Group B ancestors in the vicinity of the swamplands of southern Mesopotamia.  There, they continued to hone the communication skills and social skills that they needed in that very special environment.  In particular, they needed to evolve both socially and genetically so as to extract the maximum value from the percussive signals that they used for tracking down hippopotami in the swamplands.  This involved the invention of ways to transmit more precise information by means of percussive signals and to promote cooperation among hunting groups that often spoke mutually unintelligible languages.

​

Percussive signals enabled our ancestors to organize and cooperate on a larger scale than hominins who had moved elsewhere after the "bottleneck" in the population of our Group B ancestors/  Even more crucial than the advantage that use of the signals provided in tracking the movements of hippopotami, use of the signals forced our ancestors to develop a special talent for decoding information that is coded sequentially.

​

This is because spoken language had almost always been combined with gestural language, and our ancestors of the time had up until then never developed the capability of expressing three-dimensional concepts except by combining their spoken language with a gestural language.  But a language based on percussive signals cannot be combined with gestures, because when people communicate by means of percussive signals, they are often at some distance from each other, and often cannot combine the percussive signals (which are coded sequentially) with gestures (which can more easily convey three-dimensional information).

​

Spoken language, like percussive signals, is coded sequentially.  Most languages are built out of phonemes, words, phrases and sentences; and strings of phonemes constitute words, strings of words constitute phrases and sentences, and strings of sentences constitute a message.  Since two people who are communicating by speaking to each other are generally in close proximity to each other, and can see each other, they can easily combine their spoken language with a gestural language.  Thus, it had never before been necessary for our ancestors to convey three-dimensional concepts by means of just their spoken language, without the use of gestures.

​

A gestural language can easily convey spatial concepts by hand or body movements, while spoken language, without gestures can only be coded sequentially.  Percussive signals, on the other hand, were used mainly for long-distance communication, and it was often impossible to accompany the signals with three-dimensional gestures.

​

It was impossible, at first, for our ancestors to convey as much information as they would have liked to convey by the percussive signals.  This is because they had simply never evolved the mental equipment to code three-dimensional concepts in a sequentially coded message.  Indeed, even modern people are unable to do so if they are born deaf, or for some other reason have little or no exposure to a modern language.  But three-dimensional concepts are so important that in the course of hundreds of thousands of years our direct ancestors evolved this important capability -- especially those who stayed in the swamplands of the Middle East until relatively recent times.

​

The use of percussive signals, especially those that could convey three-dimensional concepts (like the movements of people and animals) was crucial for coordinating large-scale hunting expeditions.  This was especially true when the hunters did not all speak the same language -- the percussive signals often served as a lingua franca for people who could not understand each other's spoken languages.

​

Hand axes in Arabia

​

Around 400,000 years ago rainfall increased greatly on the Arabian Peninsula.  Hippopotami from the swamplands began to forage in the wet grasslands that then separated the swamps of southern Mesopotamia from Africa, and some of our ancestors followed the animals until they found themselves in Africa.  At the same time, there was an influx of other Group B immigrants from the Levant, who crossed the Arabian Peninsula somewhat to the north.  They met in Africa and joined forces.

The Group B immigrants from both forces were so outnumbered by Homo heidelbergensis that they disappeared into the Homo heidelbergensis population without changing it much.  Our ancestors did, however, leave some hand axes on the Arabian Peninsula for archaeologists to find.

​

A few thousands of years after this wet period, rainfall declined on the Arabian Peninsula and dry conditions again prevailed until wet conditions re3trned about 320,000 years ago.  During this second wet period, hippopotami and other animals again wandered across the Arabian Peninsula, and more of our ancestors followed them into Africa, and they left some of their hand axes and other stone tools along the way.  This time it was different, because they were met there by only a few Homo heidelbergensis, and our ancestors took over Africa.  They picked up DNA from Homo heidelbergensis, but most of the functional DNA acquired from them was quickly eliminated from their genome.