I’m a Neandertal … Get Me Out of Here!
I’m a Neandertal … Get Me Out of Here!
When being King of the Jungle was an evolutionary dead-end
Last week I read this:
This large-scale, multi-millennia-spanning comparison made it more straightforward to monitor ‘introgression’ of Neanderthal-derived sequences into the modern human genome. The results indicated that Neanderthal-derived genetic contributions in the modern samples could be traced to a single ‘pulse’ of gene flow starting roughly 47,000 years ago — more recently than originally projected —and spanning some 6,800 years, ending around the same time that Neanderthals were nearing extinction.
“Neanderthal–human baby-making was recent — and brief”, Nature, Michael Eisenstein, May 24, 2024
It seems that every new anthropology paper forces us to rethink our origins and the contribution that other hominids made to who we are. An earlier publication told us this:
As some of the first bands of modern humans moved out of Africa, they met and mated with Neandertals about 100,000 years ago—perhaps in the fertile Nile Valley, along the coastal hills of the Middle East, or in the once-verdant Arabian Peninsula. This pushes back the earliest encounter between the two groups by tens of thousands of years and suggests that our ancestors were shaped in significant ways by swapping genes with other types of humans.
This is not contradictory. The 2016 study concerns the first encounter between Homo neanderthalensis and Homo sapiens when the latter emerged from Africa only to die out later. We know they interbred because their DNA was preserved by neandertals1 that went on to head Eastwards. The paper also says that H. sapiens carried neandertal DNA across the world. This must be from encounters that are known to have happened between 35,000 and 50,000 years later after more H. sapiens came out of Africa. The latest preprint2 concerns the 30,000 years leading to neandertal extinction when they coexisted with another wave of African migrants. It was in the last 6,800 years of the neandertal era when most interbreeding took place.
On Selection
By the neo-Darwinist definition:
Natural Selection is random mutation followed by non-random selection
I suggest ‘non-random deselection’ for reasons I will explain, and as you will see, it’s the same thing.
Mutations are infrequent in humans (approximately one per ten thousand genes per generation) and as observed in the preprint, 7000 years is almost insignificant on the evolutionary scale. The rapid changes in the human genome are difficult to explain without recognising that natural selection was supercharged by the interbreeding of two species, who parted ways at a common ancestor, probably 130,000 years earlier.
Speciation
Speciation occurs when populations become geographically separated and each group is forced to adapt to different environmental pressures until they evolve as distinct species or sub-species.
Neandertals and H. sapiens had been mostly isolated from each other for a hundred thousand years prior to meeting up again to coexist for around 23,000 years. Then came the aforementioned ‘pulse of gene flow’, which resulted in a confluence of genetic traits, harvested from two separate evolutionary pathways. That the total and final interbreeding opportunity spanned a period of thirty millennia, is likely to be a source of some of the ambiguity in our ancient lineage, not that it is the only example of where different species of hominid interbred.3
Natural and ‘Artificial’ Selection
We know that selective breeding has the effect of implementing phenotypic changes quickly. Wolves were domesticated between 15,000 and 20,000 years ago and later those canines became what we call dogs, but the modern breeds we have today did not exist in the 1850s, when Gregor Mendel was carefully conducting his pea experiments. By that time, crop hybridisation had been practiced for around 9,000 years, but Mendel was seemingly the first to have considered that there was a mechanism of trait-inheritance to be decoded.
This breeding for specific traits is often called ‘artificial selection’ by evolutionary biologists and it’s easy to see why. One of my objections is that humans and human agency are also natural and I would prefer ‘augmented natural selection’ were it an option. It would provide a broader category for the impact of human agency (whatever you think that is) on natural selection. This will be explained later in this piece.
Natural Deselection
My definition of ‘natural deselection’ is therefore functionally identical to that of ‘natural selection’. It is just a change of emphasis from the selection of beneficial traits, to the elimination of harmful ones, with the added clarification that neutral traits are not indiscriminately removed, viz.
Local Definition #1: Natural Deselection (proposed alternative)
Natural Deselection is random mutation followed by non-random deselection of survival-negative genetic mutations
Note: survivability concerns the probability that a given phenotypic characteristic will impact an organism’s ability to survive long enough to reproduce.
It takes a long time for generations of phenotypes to respond to environmental pressure. Mutation rates are low because DNA/RNA transcription is a high fidelity process, but before any changes can be deselected (or de facto ‘selected’), it seems obvious to me that they must first accumulate to produce a delta in survivability. I suggest that there must be small changes that make no phenotypic difference and are therefore tolerated below the threshold of being deselectable.
Local Definition #2: Sub-Deselectable Change
A genetic mutation that results in zero or negligible phenotypic change and has no negative or positive impact on survivability is not specifically deselectable
Thus I propose that -
- the characteristics that are left after survival-negative changes have been eliminated have a chance of being replicated in the succeeding generation.
- the presence of an immediate survival advantage is not necessary for a mutation to survive, but it must at least be survival-neutral, i.e. not detrimental to the survivability of the phenotype in the generations that follow.
- some of these ‘neutral’ variations provide suitable foundations for a sequence of mutations that stack up into phenotype survivability benefits.
- it is compatible with any explanation for the phenotypic diversity in humans. It is perhaps this genetic variability that provided humans with a broad base from which parallel developmental options are put to the test of survivability.
- deselection is unambiguously a mindless process without need of agency.
Nothing I have said detracts from the marginal gains argument; Richard Dawkins argues, a blurry perception of an outline shape by a primitive eye provides survival advantage over having no eyes.
I think many take ‘non-random selection’ to mean that any mutation that provides no survival benefit immediately withers on the vine. This is mistaken, because were that the case, human phenotypes would tend to become more homogenous due to an inbuilt intolerance to genetic variation. The reality is that an a incredible amount of biological diversity survives in the modern human population. Distinctive facial features don’t tend to result in differential mortality rates, although, it might influence reproductive attractiveness across many generations.
Additionally, how would it be possible to selectively breed certain traits in or out of a population of animals, without genepool trait diversity? It should be noted that humans don’t tend to breed animals for survivability traits, so we might also call those efforts counter-evolutionary, and there’s a euphony to it.
Left to natural selection, a phenotypic change may provide incremental advantage or disadvantage, it should be obvious that it is likely to take many generations for a material impact on survivability to accumulate.
For example, vasovagal syncope is a condition that causes the carrier of that gene to faint in response to a trigger, but it only represents a tiny additional risk to an individual’s survivability. Yet over a sufficient number of generations an individual who hasn’t bred yet will inevitably faint in a perilous situation, for example when driving a car or climbing a cliff face. That would be a non-random (i.e. probabilistic) deselection of that particular trait copy.
A Pinch of Saltationism
Not having a complete fossil record does not prevent us from inferring what might have been in the gaps.
Saltationists (and creationists) claim that incremental genetic change cannot explain complex biological structures. They often express incredulity at the idea that random mutations could provide the building blocks for lifeforms and frequently harbour the unrealistic assumption that the fossil record should be more complete than it is. Apparently, accepting incremental change and reconciling the ‘missing links’, gives them some difficulty. Saltationists therefore argue that taxonomic gaps are explained by evolution plateauing for long periods that are ‘punctuated’4 by brief explosive episodes of evolution. I am not clear on what the supposed mechanism is for that.
I have proposed that there would be a threshold of accumulated genetic change to be crossed before it produced a phenotypic effect. This would be difficult to decode in the genome and perhaps undetectable in the fossil record. Yet this is still gradualism.
Perhaps the conventional definition of natural selection would have to be amended to make it self-evident.
Saltation- Resistant Definition of Natural Selection
‘Natural Selection is random-mutation, followed by random-accumulation of genetic changes, followed by non-random selection’.
Apart from my preference for ‘deselection’ I see nothing wrong with that because it clarifies the point that natural selection is acting on failure and not neutrality or benefits.
However, in my ‘non-random deselection’ construction, this problem goes away without further modification. Firstly, because it becomes obvious that the accumulation of neutral genetic mutations are inevitable (it is not going to be arbitrarily removed by ‘selection’) and secondly, the more of these ‘mutation stacks’ there are, the greater the likelihood that some will build phenotypic effects that alter survivability, one way or the other.
Divergence and Convergence
So genetic changes would be invisible in the fossil record while their influence is ‘sub-phenotypic’. Although it’s unrealistic to expect there to be a complete record of speciation, some of the gaps might be explained by a process of what I am going to call, ‘divergence and convergence’. This would only apply to species that maintained sexual compatibility of which hominids are an example, viz.
Divergence: geographical and environmental separation resulting in divergent evolutionary paths and speciation. Since environmental conditions are geographically dependent, it follows that environmental pressures drive the phenotypic adaptation of geographically captive populations. Yet those pressures on resources and habitability can also drive a more immediate response in the form of migration i.e., where it is an option.
Migrations are always towards better prospects but resilience comes from being challenged by the environment. Migratory birds flourish because they can avoid the extreme seasonal changes that must be endured by geographically tethered creatures. Land animals grow winter coats, store food or hibernate to withstand the environment but birds adapted by evolving the means to escape it.
Convergence: sexually compatible species are drawn towards favourable environments where co-existence is supported and interbreeding is enabled.
In the hominid example the benefits of diverged evolutionary paths (each more than 100,000 years long) had the opportunity to mix. Over a further period of 30,000 years these two species would have been very aware of how to live alongside each other, and in some cases, interact or exploit. These were families or tribes and the markers of neandertals in our genome might suggest long periods of peaceful coexistence as per the recent article:
Relations between a good number of humans and Neanderthals (Homo neanderthalensis) must have been cordial: almost every living individual not of African ancestry carries genetic remnants of past pairings between the two species.
Neanderthal–human baby-making was recent — and brief, Nature, Michael Eisenstein, May 21, 2024
Contrary to how they had been previously characterised, neandertals had technologies of their own including hand tools, weaponry plus culture and art. Might we expect to find that the technologies used by both species would have converged as they created hybrid families? There is evidence for that but the conclusions drawn from it seems quite subjective.
Whatever we might think of the sharing of technology the fact that one species survived while the other became extinct suggests there were differences in living strategies and/or biological survivability. Perhaps there are credible alternative explanations to ‘cordial relations’ between different species of hominid.
Competition and Arms Races
In nature there is competition for every resource and for everything that is eaten there are other animals that wanted to eat it too. The competition between prey and predator is for the right to reproduce. The cheetah may kill the gazelle but if the gazelle escapes it puts the cheetah’s survival at risk by denying it a meal - they are mortal enemies because the threat is bidirectional. Faster animals apply evolutionary pressure so only the fastest of their enemy survive to reproduce. This is called an ‘evolutionary arms race’.
A Hominid Arms Race?
We know that neandertals became massively outnumbered by H. sapiens but this may not have mattered while groupings were small and dispersed. Individually neandertals were more physically powerful and it’s always the case that ‘predators’ tend to be in the minority - a term I use advisedly. In conflicts of equally matched numbers, it seems likely that a neandertal attack on H. sapien groups would be successful more often, than the reverse case.
Any competition for resources would have exacerbated conflict but I will take this one step further. Could the two species have been locked in an arms race where neandertals were the ace predator and humans the more plentiful prey? There is evidence that neandertal carcasses were sometimes dismembered and de-fleshed by their own communities. One possible explanation is cannibalism in which case it is unlikely that they would be squeamish about eating H. sapiens if the opportunity arose.
During a period of climate change where temperatures dropped significantly, it was the neandertals that apparently found it difficult to get adequate nutrition, and it seems that resources became so scarce that they resorted to smashing up bones to access the marrow. This has led some academics to speculate on the extent to which neandertals were able to create fire but there is evidence in caves that leave little doubt that at least some of their groups did.
Some experts have mooted that their use was opportunistic after a lightening strike or a bush fire. If true, neandertals without this technology would have been at a survival disadvantage by being unable make a fire, to keep warm or cook food. As a contributor to neandertal extinction it seems dubious given the use of fire goes back to Homo erectus. Not to have witnessed fire-making would seem incompatible with interspecies cooperation or proximity.
… the simple presence of burned bones and stones or localized areas of charred soils are not sufficient to prove that hominins were actively producing fire. Before 1 million years ago, sparse evidence from some African sites could suggest that hominins were opportunistically harvesting fire from naturally kindled blazes; rather than practicing truly operative fire making. However, a multidisciplinary study from the Wonderwerk Cave in South Africa reports convincing evidence for intentional burning in a controlled archeological context dated to 1 million years old.
“How Prehistoric Humans Discovered Fire Making”, Popular Archeology, Deborah Barsky, May 29, 2024
Balance of Advantage
The consensus is that neandertals were out-maneuvered or overwhelmed by early H. sapiens but what were the contributory factors to the balance of power? As previously noted, neandertals would likely be physically dominant amongst hominids on an individual basis, so why didn’t they prevail? Perhaps it forced H. Sapiens to develop defensive strategies or form larger groups. Besides that there were ways in which neandertals were at a biological disadvantage to H. sapiens.
Greater infant mortality
Shorter life expectancy
Consequently.
Lower and less stable populations
There is evidence that some neandertals were inbred, so partner availability must have been low, possibly due to high infant mortality and death of breeding age females in childbirth. Linear regression would probably show that extinction was inevitable, but also, that it could be delayed by interbreeding; that might be why it became more frequent during a relatively short period.
I speculate that a shortage of food and inability to produce viable offspring at a rate sufficient to sustain the population may have caused neandertals to view H. sapiens as a resource to be exploited.
My Question
What would have happened to inter-species coexistence if H. neanderthalensis males preferred H. sapien females?
If neandertals raided H. sapien groups to capture females they would presumably have to kill males in the process. Is it conceivable that they would have any taboos about eating them? This would make H. Sapiens the prey of neandertals in two senses.
A possible by-product of Hominid cannibalism may have been disease. A form of TSE (Transmissible Spongiform Encephalopathy) may have terminated many neandertal familial lines. Looking for signs of those prions in the neandertal genome, might be worthwhile, assuming that would even be possible.
Prey don’t organise themselves to fight back against a predator, but H. sapiens would be atypical in that regard, having the capability to turn being persecuted into a confrontational reproductive arms race. By becoming a threat to the far more numerous H. sapiens, neandertals may have created an enemy that would further impact their own sustainability.
An asymmetric sexual-preference for plentiful H. sapien females might therefore explain a great deal. I venture to suggest it would address everything that the assumption of cordial relations seeks to explain.
Conjecture: Asymmetric Interspecies Attraction
If both neandertal and H. sapien males favoured H. sapien females the result would be sexual competition
This preference would not stop neandertal females from having children, but they would be in competition with females that had more success with childbirth and rearing, making neandertal mothers less likely to have grandchildren. A preference to breed with H. sapien females would cause neandertal genes to be diluted in the hominid genepool as later neandertal generations consisted of increasing proportions of mixed species.
Augmented Natural Selection
Previously I suggested ‘augmented natural selection’ as an alternative to ‘artificial selection’ (i.e. selective breeding), but the reason was not only to replace a term I don’t particularly like but to introduce a broader category of selective pressure, that includes ‘selective breeding’ and sexual attraction.
The preferential selection of traits drives sexual attraction but ultimately that’s about survivability through reproduction. Yet as with the selective breeding of other animals the common factor is having agency in the equation. Could a shift in neandertal mating preferences set them on the path to selectively breed themselves out? Before discussing that let’s look my local definition of Augmented Natural Selection to characterise both selective breeding and sexual attraction.
Local Definition #3: Augmented Natural Selection
Augmented Natural Selection is the acceleration of phenotypic change under natural de/selection via elevated selection pressure under the action of agency
Consider the plight of the English Bulldog. It has health issues with breathing, skin, eyes and joints including hip dysplasia. The dominant factor here is not survivability, but human intervention, thus short-term intention renders long-term survival pressures irrelevant. Phenotypic changes are responses to environmental pressures which are far higher and compounded when the environmental context includes human agency.
Sexual Attraction as a Selector
High selection-pressure brought to bear on reproduction by personal preference in choosing a mate. It is no longer a simple matter of which suitor can batter the other to death with a mammoth tusk. Yet primal instincts still influence modern preferences.
People who fit the popular notion of attractiveness tend to get together and have babies that grow up to be conventionally attractive too. Something similar can be said of athletes and the sporting potential of their children.
What would be the unconscious process by which neandertal males developed a preference for H. sapien females as I propose? Could it be because they had better success at carrying children that were more likely to reproduce? Did H. sapien females find the dominant neandertals more attractive?
A diminishing population would only reduce partner availability, genetic diversity and indeed as already noted, some neandertals were inbred. Lack of reproductive sustainability would open up generational gaps making them vulnerable but also increasingly desperate and dangerous to H. sapiens. Perhaps this created an uncontrollable biological need to interbreed with H. sapiens for the survival of their genes. The result would be that an increasing proportion of neandertal progeny would be of mixed species.
That neandertal genes survived suggests that many of these offspring were raised ‘cuckoo-like’ within H. sapien families. At some point neandertals would have become less of a potent threat, and more of a minority nuisance and perhaps it was after a period of maximum interbreeding that H. sapiens drove them to their final extinction? Could this also have been what happened to the first wave of the H. sapiens at the hands of the East travelling neandertals?
There are many potential contributors the neandertal extinction, but in forming my conclusion that one factor was interbreeding, I had thought about alternatives where sexual relations were unrelated to extinction.
Interbreeding Scenarios
Scenario #1: the two species collaborated and in some locations integrated. Neandertal communities that did not integrate perished. This is the scenario that seems to be generally assumed.
Scenario #2: H. sapiens overwhelmed neandertals by numbers and sometimes had sex with the females. They may have taken neandertal females captive and bred with them.
Scenario #3: neandertals occasionally overran H. sapiens and impregnated the females. They may have captured females too but what is clear is that at least some of those mixed species offspring must have been born into H. sapien communities otherwise their genetics would not survive in modern humans. We might call this ‘the cuckoo effect’.
Scenario Summary
Scenario #1 seems to represent the consensus but it is the one I find least likely. I do not deny that over 30,000 years some consensual liaisons would be inevitable but then so would conflict and rape. To claim that interbreeding was due to social cooperation is the least parsimonious explanation. I find Scenario #2 unconvincing because H. sapiens were not suffering from a shortage of potential partners in their own species at that time. There would be no underlying survivability need for finding neandertals attractive and interbreeding would be contrary to the genetic reproductive agenda.
As with humans today, conflict at one geographical area does not preclude peaceful co-existence in others, so there is no inconsistency in there being some of both. Yet in times of scarce resources (such as the period that led to the neandertal extinction) it seems unlikely that H. sapiens would knowingly take neandertals progeny that were not born into their community.
I propose Scenario #3 because the availability-squeeze on resources, food and sexual partners at that time would make conflict inevitable. The cuckolding of H. sapien males during raids would be an explanation for neandertal genes surviving in modern humans. The capture of H. sapien females may have sustained some neandertal groups until they effectively bred themselves out of existence.
The Asymmetric Interspecies Attraction would result from the fact that it would not be in H. Sapiens genetic interests to breed with neandertals because they had already stabilised their populations. Conversely, interbreeding would be natural selection’s way to preserve neandertal DNA and this exact mechanism may have operated on H. Sapiens when they first left Africa to stumble into neandertal territory 100,000 years ago.
Conclusion
Genetic sequencing continues to reveal more about neandertals. Their migrations and interbreeding provide clues about the pressures they were under and also those they exerted on other hominid species. I suggest the interactions between the hominids were predominantly fractious. Resource scarcity is more likely to result in conflict than collaboration.
During a period of more that 30,000 years of coexistence there would likely be many localised interludes of cooperation. But if as I suggest, they are likely to be uncharacteristic, it wouldn’t be very useful as a main focus of study. That there is evidence of extensive interbreeding in a 7,000 year window only tells us that inter-species sex took place. It does not follow that it was cordial or even consensual.
Extinctions occur when species are unable to adapt to environmental changes quickly enough to survive. That neandertals and H. sapiens appear to have had different responses to resource challenges doesn’t suggest widespread integration. I propose that amongst H. sapiens there were a minority who carried neandertal genes, but once the neandertal communities failed around 40,000 years ago, the phenotype was quickly annihilated. This was followed by a long process of genetic dilution that continues to this day.
I have adopted the spelling ‘neandertal’ over ‘neanderthal’ except when quoting those who use the alternative.
“Humans mated with Neandertals much earlier and more frequently than thought: New study pushes back the first liaison by tens of thousands of years”, Science, Ann Gibbons, February 17, 2016, which looks at instances of interbreeding between other hominid species.