BOTTLENECK

Groups A, B, C and D

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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 Homo antecessor 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.  The existence of these two groups is inferred from genomic analysis of modern human populations.

At the same time that Group A and Group B were moving south, other archaic humans were surviving in refugia in Eurasia, and descendants of two of these groups survived in Eurasia and Island Southeast Asia until they were overrun by expanding populations of  I will divide these northern survivors into two parts, based of genetic clustering of their descendants' genomes.  One of these parts survived in southern Europe, most likely on the Italian and/or Iberian Peninsulas and I will label them Group C, and meanwhile another part survived far to the east, perhaps in South Asia.  I will call this South Asian part Group D.  The existence of these two groups is postulated, based on the later existence of Homo neanderthalensis and Homo denisova, who are postulated to be partially descended from them.

We will initially follow the fortunes of Group A, because we are primarily descended from them.

Bottleneck of Group A population

Those in Group A barely survived, and we have already mentioned their population bottleneck as their total effective population was reduced to about 1,200 individuals, and their numbers remained low for about 100,000 years.  They survived and eventually began to increase their numbers by learning how to bring down the largest animals in the swampy regions in southern Mesopotamia:  Hippopotami.

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 took 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.

As is true today the swamplands were fed by rivers that flowed south through Mesopotamia, and the flow in those rivers varied throughout the year.  following each seasonal flood, as the waters receded grasses grew abundantly along the banks of the rivers.  Herbivores grazed on the grasses and hunters moved north to hunt them, and eventually encountered Huge elephants, much larger than today's African elephants.  These elephants migrated mainly along a rift valley that followed the course of today's Jordan Valley but continued far to the north.   Thus, members of Group A moved into the Rift Valley and began hunting the giant elephants, as dangerous as they were -- a single giant elephant, like a single hippopotamus -- could feed a lot of people.

Percussive signals

Let us now return to our narrative about our Group A ancestors in the vicinity of the swamplands of southern Mesopotamia.  After the ancestors of  and  had separated themselves from our own ancestors and moved off into Eurasia our own direct ancestors 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.

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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 A 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.

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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 cannot combine the percussive signals (which are coded sequentially) with gestures (which can more easily convey three-dimensional information).

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As I noted, 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.

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A gestural language can easily convey spatial concepts by means of hand or body movements, while spoken language, without gestures, can require visualization skills which our ancestors did not have.  This shortcoming in visualization arose when they needed to convey three-dimensional information by means of sequentially coded information that was not accompanied by gestures.  This is because gestures, which consist of hand and body movements, can be three-dimensional in their very nature.

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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 often unable to do so if (in the absence of a sophisticated gestural language) they are born deaf, or for some other reason have little or no exposure to a modern language.  But the conveyance of three-dimensional concepts by means of percussive signals was so important to our ancestors that in the course of hundreds of thousands of years our Group A ancestors who stayed near the swampland until relatively recent times did evolve this important capability.

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The use of percussive signals 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 served as aa lingua franca in such cases.

Fine tuning of the vocal tract

Concurrently to the long process of developing an ability to use percussive signals more effectively our ancestors refined the effectiveness of their vocal communication.  Use of percussive signals freed our ancestors of their need to use their voices for intragroup and intergroup communications.  For instance, males had earlier signaled their presence with their voices while females had signaled their sex and peaceful intentions with their voices as they approached the vicinity of camps.  Instead, our ancestors communicated at a distance by using percussive signals, and used them to establish friendly relations with neighboring groups and even combine their hunts at a time when the groups were small and often spoke mutually unintelligible languages.

This use of percussive signals for communication at a distance allowed evolution to fine-tune vocal communication in order to improve face-to-face communication.  As it became less necessary for our ancestors' voices to carry over long distances evolution discarded the vocal membranes that sat above the vocal cords and created chaotic vibrations in the layer of air above them.  These vibrations caused the voice to carry well but made it impossible for earlier humans to produce the variety of speech sounds that our direct ancestors became capable of making.  This, combined with other adjustments in the vocal tract, and together with further development of the brain, contributed to the superiority of our ancestors' face-to-face communications

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.  Eastern Africa was not to the liking of the new immigrants, so most of them turned around and returned to southern Mesopotamia, but some of these newcomers explored Africa, looking for something better.

Some of the immigrants explored to the south, where they found both hippopotami and elephants.  Likewise, others found their way into the Nile Valley and found hippopotami and elephants there.  But the indigenous people in both places were so numerous and hostile that the newcomers settled for west-central Africa, where patches of grassland had opened up with drying of the climate.

We know about this early influx of proto-Homo sapiens into Africa because our ancestors left calling cards on the Arabian Peninsula -- hand axes that were handy for slicing up megaherbivores but were unnecessarily heavy when chasing down the smaller game of Africa.

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A few thousands of years after this wet period, rainfall declined on the Arabian Peninsula and dry conditions prevailed until a hundred thousand years later, when wet conditions again returned to the Arabian Peninsula -- about 300,000 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 left their calling cards along the way -- hand axes that were somewhat smaller and sharper than those that had been left there a hundred thousand years earlier.  The hand axes might have been somewhat smaller because elephants, for which the larger hand axes had been designed, were not as plentiful as they had been a hundred thousand years earlier.  The smaller hand axes had been designed primarily for slicing up hippopotami, which were considerably smaller.

This time it was somewhat different, because eastern Africa had become very dry, and this had emptied the region of the indigenous population, and the new immigrants found a population of archaic Homo sapiens already occupying eastern Africa.  (These archaic Homo sapiens had returned to the region from Central Africa.)

Although the region was very dry, there were many lakes in the Rift Valley of eastern Africa and the rough country there provided many opportunities for trapping and slaughtering animals.

Strategic hunting

The archaic Homo sapiens of Africa, like their ancestors in the Middle East and like those who had moved into Central Africa, hunted in quite a different manner than the indigenous people of Africa.  Their ancestors had found the herds of hippopotami in the Middle East scarce and elusive, and they had also found the of smaller game of Central Africa likewise scarce and elusive, so they had adopted the tactic of taking ownership of those that they found, and culled the herds carefully -- killing only those that were in their prime.  And leaving room for new generations.  Finding game scarce in relatively dry eastern Africa. they did the same there, whereas the indigenous people further south and in northern Africa wastefully trapped and slaughtered whole herds when they had the chance.  This made it possible for Homo sapiens to continue to prosper when climate change reduced the animal populations on which they depended, and may have helped them to take over all of Africa.

Sudden appearances around Africa

Our Homo sapiens ancestors -- by this time most likely carrying some DNA picked up from the Broken Hill Homo -- also began showing up in other parts of Africa about 300,000 years ago because their population had been increasing in Central Africa throughout the hundred-thousand years between the two influxes into Africa.  The earliest evidence of this population buildup appears first in the fossil record -- almost simultaneously -- in Morocco, at the western end of northern Africa, and Ethiopia, at the eastern end.  Along with the fossils archaeologists found their stone tools, which were smaller and lighter than hand axes but convenient for butchering the gazelles and zebras that they found in Morocco and other animals that archaic Homo sapiens found along the Rift Valley at the eastern end.

From then on, archaic Homo sapiens begin to show up in the archaeological record at various sites in Africa -- they spread out in every direction from Central Africa, most notably to the northeast into the Nile Valley and to the southeast into what is now Zambia.  They undoubtedly pushed aside the indigenous Broken Hill Homo in order to take over that valuable territory but also interbred with them.

Stone-tool innovation flourished especially in Central Africa. because that was where their population was centered, and they proved to be much better than the African Homo heidelbergensis (the Broken Hill Homo) at maintaining social and trade networks that spread innovations throughout their African population.  This was partly because of their ability to communicate while still at a distance, by using percussive signals.

Evidence has been found in South Asia of the use there of advanced tools and weapons as early as 200,000 years ago.  This may mean that there was an early Out-Of-Africa movement about that time, but it might also mean that some archaic Homo sapiens moved into South Asia directly from the swampland regions of southern Mesopotamia.  In either case they could have been subsequently wiped out or seriously impacted by the Toba super-volcano, whose eruptions in Indonesia buried vast areas in South Asia and elsewhere in deep layers of poisonous ash.

Homo neanderthalensis

As noted above, after the Group B population of our ancestors emerged from their long population bottleneck some part of this population moved into the Rift Valley that followed the course of today's Jordan Valley.  Later on, as  populations of herbivores in the Rift Valley declined -- including elephant populations.  By 400,000 years ago elephants had become extinct in the Levant, but around that time the climate in the region changed drastically, opening pathways for both herbivores and humans to move south into Africa as well as north into Eurasia.  Consequently, many members of the Group A humans in the Levant moved north into Eurasia where they interbred with the archaic humans whom we have labeled Group C.  This cross-breeding led to aa new species which is known to us as  Homo neanderthalensis.

We know something about who they interbred with there because archaeologists have found fossils left in the Atapuerca mountains of Spain, dated at about 124,000 years ago, that are likely representative of the  hominins in Eurasia with whom the immigrants from the Levant interbred about 400,000 years ago.  Moreover, we have extracted DNA from them and determined that unlike Homo neanderthalensis their mitochondrial DNA was quite different from that of modern humans.

While the fossil finds in the Atapuerca Mountains were not accompanied by any of their stone tools, archaeologists have found a site in the central Italian Peninsula, dated to 404,000 years ago, where hominins butchered a giant elephant. This butchery involved a novel technique that involved the insertion of small sharp chips into a wooden club which was then used to pound and shred the flesh of the elephant, cutting it away from the bone.  Small hand-held chips were also utilized in butchering the elephant.

This butchery may have been done by members of the Group C population that later interbred with the Group A population that migrated from the Levant, or might have been done by members of the interbred population that were ancestral to Homo neanderthalensis.

For the most part fossils of Homo neanderthalensis have been dated to about 300,000 years ago or thereafter, indicating that it took about 100,000 years for the species to populate Europe in large numbers.  This conclusion is borne out by genetic studies, which found that Homo neanderthalensis entered another severe population bottleneck during that interval.

Homo denisova

As rainfall increased dramatically the Middle East about 400,000 years ago it also increased dramatically on the Iranian plateau and in places to the east of there, and archaic Homo sapiens followed hippopotami and other grazing animals that moved into those regions as well.  In South Asia they encountered indigenous people there, whom I have labeled Group D and interbred with them.  The result of this interbreeding was still another species of archaic humans

This species was first identified as a new species on the basis of DNA that was extracted from fragments of fossilized bone that were found in Denisova Cave in the Altai Mountains of Siberia, and since the fragments of bone were insufficient for identifying skeletal remains (without extracting DNA from them) the species has not been named.  Still, the species needs a name, so I will call them Homo denisova.

XXXXXXAfter their population had rebounded from the population bottleneck 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.XXXXXX

XXXXXXSubsequently, there were two more splits, within 60,000 years of the movement of early Homo neanderthalensis 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).XXXXXX

XXXWhile 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.XXX

XXXHippopotamiXXX

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xxxCarp fish and elephantsxxx

XXXMeanwhile. 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.XXX

At Gesher Benot Ya'akov in northern Israel, along the route of this ancient migration trail, archaeologists found evidence of campsites which they dated to 780,000 years ago.  There, the early humans found everything they needed:  driftwood from a lakeshore that they gathered that could feed their fires; various plant foods including fruits, nuts and seeds from lakeside vegetation; carp fish that they caught from the lake; and various grazing and browsing animals.  In particular, they sometimes came across the giant elephants.  From nearby outcrops of flint, limestone and basalt the early Levantines harvested the raw materials for stone tools.

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 might have recombined with ancestors of Homo neanderthalensis.

 Fossils of early or pre- Homo neanderthalensis have been found in a cave in the Atapuerca Mountains of northern Spain and dated to about 425,000 years ago.  Other evidence of pre-Homo neanderthalensis activity in the Atapuerca mountains include hunting and butchering tools associated with evidence of the hunting and butchering of Bison.

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 meat was shredded away from the bones 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.

Lifestyles of our sister species

Very little is known about the lifestyles by which our sister species  Homo denisova lived and evolved in the course of their nearly 400,000 years of existence or indeed what caused their eventual demise or disappearance as separate species.  As for why they are no longer with us, many theories have been proposed, but one explanation stands out as an undeniable partial if not total reason for their disappearance:  Their numbers were so small that they were absorbed into our own genome without changing it much, and subsequent evolution eliminated those features of their genomes that were incompatible with ours, or with our own ways of life.

As for their own ways of life, almost nothing is known about the lifestyles of Homo denisova because we only have positively identified as belonging to them are a few traces of their DNA.  Still, some clues can be gleaned from the distribution of their remains, which consist, broadly of bones and teeth from which their DNA has been extracted, skeletal remains that have been identified as possibly belonging to them, and the DNA that they have left behind in our own modern human genome.

The bravery of horned animals

Homo neanderthalensis were horned animals in the sense that they bore pointed horn-like weapons, and so were elephants.  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 favorites 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 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 was 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 were found in the cave, along with the bones of other animals.

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, most likely, belonged to the same species whose DNA was found in Denisova Cave, and I will call them Homo denisova.

Interestingly, the oldest layers of dirt in the cave, dating to as old as 400,000 years ago contained pieces of bone from deer, wild horses, bison, gazelles and wooly rhinoceroses, along with stone tools and along with bones that had belonged to both Homo neanderthalensis and Homo denisova, proving that these ancient humans had migrated into Siberia by that time.  Some of the animal bones had cut marks indicative of butchering, and some were burnt.  All of this proves that the ancient humans actually lived in the cave, which is and was ideally located for hunting activities. 

DNA of Homo denisova 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 or nearly al 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.​

The pattern of inheritance of Homo neanderthalensis DNA that is found in modern humans indicates a set of circumstances that may have a bearing on the demise of  about 400,000 years ago.  Just prior to that extinction of our cousins a lot of interbreeding took place between us and them,  but much of it did not go well.

Interbreeding problems

For instance, the complete lack of Homo neanderthalensis mitochondrial DNA in modern humans seems to indicate that it was difficult or impossible for a Homo sapiens man to mate successfully with a Hom neanderthalensis woman, possibly because the woman's ovum was too choosy, and wouldn't accept the Homo sapiens sperm.  Alternatively, there may have been an incompatibility between the resulting fetus and the body of its mother, such as the incompatibility between an Rh positive fetus and an Rh negative mother.

This got the two species off to a bad start when it came to interbreeding, but then it seems that everything did not go well in the second generation, either:  male offspring also encountered difficulty when they tried to produce offspring.  Their sperm may have tended to be rejected by Homo neanderthalensis women or by by mixed-breed women that they tried to mate with.  All in all it was a big mess.

In spite of all of this, the two species did sometimes manage to successfully mate, as evidenced by the small amount of Homo neanderthalensis DNA that is carried by modern humans.  But what happened to the Homo neanderthalensis people themselves?  They are nowhere to be found in the modern world.

Battle of the sexes

Let us follow in detail what happened when c moved into Eurasia:

Homo sapiens has been moving into the Middle East for thousands of years, so there is quite a buildup of that species in the Levant, in Anatolia and on the Iranian Plateau.   Homo neanderthalensis occupied territory north of there, in Eurasia, and both populations were migrating in random directions at a slow pace.  Population density was ten times higher in Eurasia than in the Middle East, because of the difference in climate, so the direction of migration was primarily to the north, into Eurasia.  And there, Homo sapiens tried to mate with Homo neanderthalensis.

Let us assume, for a moment, that a female Homo sapiens moves into a Homo neanderthalensis extended family to raise her brood of half-breed kids.  When these kids come of age the males will not be able to mate successfully with Homo neanderthalensis women, and will not produce any offspring.

Now, let us assume that a female Homo neanderthalensis moves into a Homo sapiens extended family to raise her brood of half-breed kids.  She is even worse off, in terms of procreation, because she will be childless.

Now, let us assume that a male Homo sapiens moves into a Homo neanderthalensis extended family and tries to raise a family of his own.  He will also not produce any progeny.

Finally, let us assume that a male Homo neanderthalensis moves into a Homo sapiens extended family and tries to raise a family of his own.  There is a reduced likelihood that his own male offspring will produce children of their own.

The upshot is that the mixed population will produce fewer offspring than expected, but not to worry:  Homo sapiens are moving into Eurasia at a rapid clip to make up the loss.  On the other hand,  Homo neanderthalensis will drift into extinction in a few hundred years.

The long, slow decline of Homo neanderthalensis

Another factor that might have hastened the decline of Homo neanderthalensis was reduced genetic diversity that resulted from another population bottleneck:  They were almost completely wiped out during the harsh environmental conditions that existed in Eurasia about 75,000 years ago, and the species seem to have survived these exceedingly cold conditions only in a refugium in what is now southwestern France.  While the species did survive in the refugium and later spread throughout much of western Eurasia, their drastically reduced genetic diversity might have contributed to their inability to compete successfully with Homo sapiens during a later period of intense cold, even as Homo sapiens survived in Eurasia and increased their numbers.