Assuming for a second that humans manage to survive that long, could we one day see multiple humanoid species descended from Homo sapiens? And although I know it would be an absurdly long time by our limited individual perceptions, about how long would one expect for it to happen?
Certainly. If civilization falls, it just takes a successful series of mutations that makes interbreeding difficult or impossible with those that lack the mutation. An extra chromosome, a mutant X chromosome that kills standard human male embryos*, etc. If civilization doesn’t fall, then we can be expected to do it to ourselves.
- The X and Y are in competition and that sort of mutation apparently occurs on a fairly regular basis on an evolutionary time scale.
The only way humans* could speciate is if a group became so separate from the general run of humanity genetic transfer stopped entirely, and even if that happens for quite a long time it still isn’t a guarantee: Australian Aboriginals and the Sami people in Finland are still compatible in all the relevant ways despite looking very different, living in very different climates, and having had no genetic contact until relatively recently. Given all that, I think it’s reasonable to assume it would take an extremely prolonged separation to ensure speciation occurs.
*(Humans can’t do sympatric speciation, which is one species diverging from another species when they both share the same range. We’re always going to look too attractive to each other regardless of the gross physical differences and cultural strictures are even less effective than the rhythm method.)
Check this thread .
Well, when the lower class is eventually driven underground, they will eventually lose all melatonin, grow tons of hair for warmth, while remaining quite agile. The upper class, by being pampered, will lose height, and intelligence. Eventually, the lower classes will feast upon the upper classes.
It’s all in this documentary.
Of course, this would make the carrier incapable of mating with anyone else (assuming that the same presumably random mutation doesn’t occur in a pair of compatible and accessible male and female). Radical saltation or “hopeful monster” mutations are so unlikely to be successful as to be essentially impossible, because even if the new mutation is functional, transmissible, and non-mortiferous it will rarely convey enough of an advantage to make a stochastic difference in the spread of the gene through the population; in other words, unless it is allows the holder to dramatically increase his or her prolificacy and the mutation can be correctly expressed in the offspring that are awarded it, it will convey no eventual advantage. True beneficent mutations and advantages due to genetic drift are relatively gradual processes, though a dramatic change in environment or other selective pressures can cause previously neutral traits to become either significantly positive or negative in terms of evolutionary fitness and result in a radical change in the population; this is known as the punctuated equilibrium hypothesis. For the most part, successful mutations are initially neutral and hitchhike along with other genes (called genetic drafting) until they find a combination of alleles that provides a good milleu for beneficial expression.
The one class of exception to this is non-nuclear chromosomes which are not subject to combination in sexual reproduction; in Animalia, this is predominately the mitochondrial DNA (mDNA), although there is other non-nuclear DNA running around the cell as well. The mDNA, conveyed by the ovum, is then reproduced in the developing organism’s somatic cells, such that any successful mutation is conveyed directly down the maternal line without modification except by further mutation. (The same is true with the Y chromosome in human males, although it is less robust and its’ epigenetic expression is controlled by other genes within the nucleus.) Such radical mutations are rarely successful, but when they are can lead to dramatic modifications in the basic reproductive behavior of the organism that are readily transmissible and expressible.
To address the question of the o.p., the species could certainly diverge, but it would require an extended period of genetic isolation via geographical barriers or artificial separation. Given the degree of genetic mixing and travel that occurs in modern society it would be impossible for this to happen “accidentally”. Strand a population of humans in a giant self-sustatining space habitat or terraformed moon for a few millennia and it is almost inevitable that some significant degree of differentiation that we would at least consider to be subspeciation; the more time, the more likely that reproductive interaction would be reduced or eliminated, leading to definitive speciation.
Stranger
Actually, it is pretty rare for speciation to take place through mutations alone. Speciation usually takes place through the accumulation of differences in gene frequencies in isolated populations that make the populations less likely to mate with one another (not ones that confer sterility). Behavioral reproductive isolation often precedes physiological hybrid sterility by millions of years; for example, many species of ducks and canids are quite capable of producing fully fertile hybrids in captivity, even though they rarely interbreed in the wild.
As has been said, the only way that humans could speciate naturally (that is, barring selective breeding or genetic engineering) is for populations to become isolated for very long periods of time. Speciation can perhaps take place in as little as 10,000 years in animal populations (though it may take millions for the process to be completed). However, since the generation time of humans is at least 20 times longer than in most animals, I would say that we would be looking at a period of 100,000 years or upward.
Just as a point of reference, how long were aboriginal Australians isolated from the rest of humanity?
Never.
You may be right about that, but could you provide more information/evidence?
What makes you think they were ever isolated? I think you need to first establish some evidence that they were. There are lots of islands that have had human populations on them for 10s of thousands of years, and few, if any, were ever isolated for any significant period of time (Tasmania seemingly being the exception).
If you want a point of reference, Tasmania would probably be the best at what we think was 10k years of isolation.
Indonesians regularly visited Australia, we have archeological evidence of semi-permanent sea cucumber harvesting and processing stations in northern Australia. However, there just weren’t many goods of interest to the Indonesians that would make large scale trade attractive.
That is almost certainly not true. While estimates vary widely as to when the main migration of pre-aboriginal peoples from the Malay Archipelago (or whether it occurred in one migration or several waves) there was a substantial period of cultural isolation prior to the Papuan people of what is now New Guinea introduced the domestic canine that came to be known as the feral dingo, ~3500 BCE. The Aboriginals developed distinct cultural, functional, and linguistic systems that have no root in Papuan, Indonesian, Melanesian, or other Polynesian culture. Any contact they had previously with the outside world prior to the development of Polynesian blue water navigation would have been sporadic at best, and had virtually no impact upon either their culture or gene pool. (I am not including the Torres Strait Islanders and other Melanesian-derived peoples as being Aboriginals even though they were indigenous at the time that European explorers arrived in Australia and were therefore lumped into the same category.) The Aboriginal peoples are themselves a highly diverse group of differentiated populations; there are easily twenty groups of linguistically and culturally distinct ethnic populations of Aboriginal Australians, though obviously coming from a limited gene pool there is not a substantial amount of genetic variety compared to, say, Africa or Eurasia, but more than is found in the Clovis-derived indigenous Americans. Depending on what you consider a credible estimate of the latest migration, the period of isolation could be anywhere from 30,000 years to over 100,000, though evidence tends to point more toward the former than the latter.
However, the gross total of genetic distinction between Australian Aboriginals and the amalgamated population of Eurasians and Africans is virtually negligible. Obviously the European settlers–who were about as separated both geographically and in gene space from the Abos as much as any population–had absolutely no problem interbreeding with and producing viable non-hybrid offspring with the indigenous people of Australia. You could attempt an argument for subspeciation based upon phenotypical expression of distinct traits, but it wouldn’t be a very strong argument insofar as those traits readily meld, i.e. skin color, facial features, adult height, et cetera all fall along a continuous spectrum rather than display discrete breakpoints. There is no practical value in making such a distinction except to reinforce prejudicial and scientifically groundless statements about “racial” priority.
Stranger
This is what I was getting at.
If c. 30,000 years of isolation didn’t result in speciation, how long an isolation would it take?
Earth is about to be destroyed. Humankind colonizes several other star systems in different directions with multiple several generation ships and once having gotten there have no particular cause to get back or to mix it up with each other; technology is focused on making a success on the planet more than improving intra-planet travel and gradually those with biology better suited for that environment reproduce more successfully.
How long until the isolated populations speciate?
Well from human evolution on Earth we can expect that within 100K years would not be pushing it too fast and if the environments would disparate enough, and the differential reproductive success for various mutations significant enough, there is no reason to believe that it could not be much faster.
OTOH, if the environments were functionally identical, or made so by technology, and any divergence of the species were due to random drift alone, then it could be a very long time indeed.
Then again, the size of the founder populations also play a role. If each new planet was founded by a small group, say a thousand individuals, then even random drift might be able to do it relatively quickly.
As I mentioned above, upwards of 100,000 years; more likely several million to reach the point where full hybrid sterility would have developed.
The Neanderthals diverged from modern humans between 400,000 and 800,000 years ago, depending on which estimates you believe. However, recent evidence indicates that at least limited interbreeding took place in the Middle East, so full speciation had not occurred in that time.
The ideal conditions for speciation would be a small founder population, that remained small for an extended period of time (and hence would lose alleles due to genetic drift) and was also under strong selection.
Quoth Colibri:
So what would a fertile hybrid of a duck and a dog look like?
(c’mon, you had to know someone would ask)
It’s not clear to me how we could conclusively say that there was no cultural contact prior to 3500BCE. There could be all sorts of contact that we are unable to detect this far removed in time.
My understanding was that we don’t know how isolated Australia was after the first arrivals, but if you have a link to some genetic studies that conclude otherwise, I’d be interested in seeing them. It seems like there is a default assumption of isolation when we really don’t have any good reason to believe that.
As for speciation, I think human populations are not going to be subject to the same “behavioral barriers” to mating that other animals might have and that the time for speciation is going to be subject to the time of physiological hybrid sterility that **Colibi **mentioned in his first post. That would tend to put human speciation in the 100s of thousands if not millions of years.