Interbreeding Between Species
From time to time I encounter the assertion that H. sapiens (and/or H. sapiens sapiens) could not have interbred with H. erectus, because they are different species. I've also been told that, "If they could have produced fertile offspring, then they weren't really different species". These fairly common misconceptions proceed from a misunderstanding of the 'biological species concept', which makes species distinctions based on fertility. Most people leave school thinking that, if two creatures can produce fertile offspring, then they must belong to the same species. I wouldn't be surprised if many teachers actually tell students that, but it simply isn't so.
The biological species concept was developed by Ernst Mayr, in 1942. Here it is, as first formulated, and quoted in Douglas J. Futuyma's Evolutionary Biology (1998): "Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups". The "reproductive isolation" can be genetic (non-fertility), geographic, or behavioral; there is NO criteria that says (as is commonly believed) that if two populations can interbreed they are the SAME species. There is NO criteria that says that two distinct species CAN'T interbreed. Consider the example of wolves, coyotes and dogs: three distinct species that can interbreed. In fact, all species of the genus Canis can mate and produce fertile offspring (Wayne et al., 1997, re: A. P. Gray, Mammalian Hybrids). This is so common, that biologists actually use a different formulation of Mayr's definition: they say, "If two populations can NOT interbreed, they are NOT the same species." That is a very different statement. Note that this is an empirical definition, and gives no guidance in regard to extinct taxons, but the bottom line is: nothing in the biological species concept contradicts the idea that erectus and sapiens could and DID interbreed.
I think it will come as a surprise to many that most scientists accept the fact that sapiens and erectus were so closely related that they could have interbred with each other. To begin with, some (probably most) scientists don't think erectus and sapiens were genetically separate species at all. They consider them developmental 'grades' within a single taxon. Here is an example of that view, from Futuyma.
"The word species, however, is sometimes used simply as a name for a morphologically distinguishable form. This is especially true in paleontology, in which a single, evolving lineage (gene pool) may be assigned several names for successive, phenotypically different forms. For example, Homo erectus and Homo sapiens are names for different, distinguishable stages in the same evolving lineage. They are chrono-species, rather than separate biological species. The two species names do not imply that speciation (bifurcation into two gene pools) occurred: in fact it probably DIDN'T in this case." [my emphasis on didn't]
Futuyma claims erectus is "human", probably because all those bad bones from Africa show such strong expression of erectus traits. The afrocentrists say they were erectus slouching toward humanity; I say the more modern-looking fossils were erectus hybridized with sapiens. BOTH views imply that erectus and sapiens were able to interbreed. In fact, the afrocentrist position, that there was only a SINGLE gene pool, is a stronger statement of their capability for interbreeding than mine. Wolpoff, and other multiregionalists, exhibit similar thinking: he maintains that erectus was "human" and evolved into modern sapiens all over the world, while the afrocentrists say that process only culminated in Africa, from whence a modern human type radiated, displacing all other 'people' without interbreeding. They don't deny those (supposedly erectus-derived) moderns and Eurasian indigenes could have interbred, they just claim they didn't.
So, nearly everybody agrees that erectus could interbreed with sapiens: multiregionalists, afrocentrists, and even me. Note, however, that some people also say erectus was a distinct taxon. In fact, Rightmire, a recognized expert on erectus, says (The Evolution of Homo Erectus, Cambridge, 1990) they were a distinct species; I even agree with him. It is interesting to see why there is disagreement on the subject. Wolpoff, and others, compare the early African and Asian skulls with the most modern ones and show that there was an increase in cranial capacity, and a morphological tendency toward some sapiens characteristics. BUT, those recent skulls are the very ones I contend are hybrid specimens! Rightmire excludes the late, Southeast Asian skulls from Ngandong for very good reasons, and shows that the rest of the series reveals NO statistically significant development toward becoming modern human. That is even with including later, African skulls that I think show some interbreeding with sapiens radiating out of Eurasia. When you get up to the recent African material, which shows significant sapiens influence, the afrocentrists claim those aren't erectus, but 'early sapiens'. For instance, they call the Herto skulls H. sapiens idaltu.
So, the real difference in viewpoint is whether: 1) erectus evolved into modern humans by a gradual process, with intRA-species gene flow (whether it occurred only in Africa or also elsewhere) or 2) erectus and sapiens interbred, founding some (tropical) modern populations, while Eurasian sapiens founded Eurasian populations, which is my interpretation of the data. None of these views preclude interbreeding between erectus and sapiens, and the multiregional position DEPENDS on it. Note how 'shifty' Wolpoff is. As a multiregionalist, he argues that "gene flow" (interbreeding) between advanced populations (who are called sapiens because of their clearly more-modern morphology) and less-advanced (erectus 'grade') specimens caused all the world's 'people' to evolve into sapiens. YET, in attempting to refute my view (that the Ngandong skulls represent hybrids between erectus and sapiens) he characterizes THAT as intER-species gene flow, as if it were not exactly what his own theory implies. Then to further obfuscate, he plays the race-card, saying my hypothesis, "raises the specter that some populations will be seen as differing because they have more genes from an extinct species". Well, yes!
If H. sapiens or s. sapiens could interbreed with erectus, then they should certainly have been able to produce fertile offspring with other sapiens, such as H. sapiens neanderthalis, or with H. heidelbergensis, which may have been a direct ancestor. Consider that wolves and coyotes have been distinct species for nearly a million years, or more than 300 thousand generations. A similar number of generations would take the human ancestry back nearly to the last common ancestor of Homo and Pan.
A final consideration is the distinguishing characteristics that differentiate the various Homo species. IF they were separated by potentially incompatible mutations, then there might have been diminished fertility between those species. However, it appears they have been distinguished by neoteny: ancestral forms were succeeded by juvenilized versions of themselves. While the effects of neoteny (such as increased intelligence, delayed maturation, progressive gracilization, and a diminution of some ancestral-adult characteristics) may be profound, the genetic changes are subtle. There seems to be little or no impediment to fertility, as the new type must have been fertile with the parent species in order to survive. Accordingly, the entire genus Homo has probably been intER-fertile, just as the genus Canis is.
Clifford Jolly, writing in the American Journal of Physical Anthropology (2001; Supplement 33: 177-204) discusses the even more apposite hybridization of hominins. He says,
''Another source of phylogenetic uncertainty is the possibility of gene-flow by occasional hybridization between hominins belonging to ecologically and adaptively distinct species or even genera. Although the evidence is unsatisfactorily sparse, it suggests that among catarrhines generally, regardless of major chromosomal rearrangements, intersterility is roughly proportional to time since cladogenetic separation.'' And, ''any hominine species whose ancestries diverged less than 4 ma previously may well have been able to produce hybrid offspring''
Four million years ago takes us back before Homo is recognized to have existed! And that is not even considering that Homo species have a longer generation time, so an equivalent number of generations would extend the potential hybridization period even further than 4 million years into the past. As an aside, this suggests that the genus Homo could have begun by hybridization. That would offer an explanation for why we are so closely related to the knuckle-walking chimps and gorillas, while Homo had bipedal ancestry. Of course, chimps and gorillas may have split off the line of descent from a common bipedal ancestor and reverted to knuckle-walking. The important point, with respect to interbreeding of species, is that hominin species separated by several million years of divergence can still produce fertile hybrid offspring.
By contrast, the divergence time separating erectus from sapiens, or the latter from Neanderthals, is much less. For instance, Krings, et al. (in DNA sequence of the mitochondrial hypervariable region II from the Neandertal type specimen, PNAS 1999) estimates that Homo sapiens sapiens and H. neanderthalsis shared a common ancestor not more than 741, and perhaps as recently as 317 thousand years ago. Afrocentrists believe sapiens diverged from erectus only a couple of hundred thousand years ago. Even if sapiens shared no common ancestors with erectus after the earliest known Homo fossils in Eurasia, 1.8 million years ago, they should still have been inter-fertile. In fact, morphological features of the Nagandong, Kow Swamp, Herto, and other skulls suggest that sapiens and erectus did interbreed and produce offspring. I contend that view is confirmed by the genetic evidence cited in Age & Origin of the Human Species, Plural Lineages in the Human mtDNA Genome , and Australian Ancestry: Implications for the Origin of H. sapiens sapiens.
In Number of ancestral human species: a molecular perspective, (HOMO Vol. 53/3, pp. 201–224) D. CURNOE, and A. THORNE directly address the question of whether recent types of Homo would have been able to mate and produce viable and fertile offspring. They say, flatly:
”All fossil taxa were genetically very close to each other and likely to have been below congeneric genetic distances seen for many mammals. Our estimates of genetic divergence suggest that periods of around 2 million years are required to produce sufficient genetic distance to represent speciation. Therefore, Neanderthals and so-called H. erectus were genetically so close to contemporary H. sapiens they were unlikely to have been separate species. Distances calculated here for H. neanderthalensis versus H. sapiens and for H. erectus versus H. sapiens are around one-third and two-thirds, respectively, of the mammalian intrageneric mode.”
Some genetic data from humans, chimps, and orangutans suggest there were genetic speciation events in Homo’s history, resulting in populations that could not have interbred with their ancestors, but not many nor recently. This type of speciation, as a result of infertility by reason of genetic incompatibility, must be distinguished from the evolution of ”type” morphology, leading to species designations such as erectus and neanderthalensis.
”Sumatran and Bornean orangutans differ by three chromosomal rearrangements but are known to be fully fertile, and common chimpanzees and bonobos differ by six chromosomal rearrangements, and although some workers regard them as distinct species (see above), they do produce apparently normal hybrid offspring (H. Vervaecke, pers. com.). Most types of rearrangements between orangutan subspecies and between common chimpanzees and bonobos are also seen in humans. This suggests that at least some of the rearrangements in humans might not represent reproductive isolation.”
But they go on to say,
”This observation is complicated by the fact that humans appear to possess even greater chromosomal instability than great apes. Humans possess a high level of chromosomal rearrangements, with 1 out of every 120 babies born being abnormal (Hook 1992). The figure rises to about 25% for 10-day old blastocysts (Gardner & Sutherland 1996). We conclude that chromosomal rearrangements were likely to have been important during human evolution, more so than among the great apes, making comparisons with them of limited value.”
”Given the chromosomal instability in humans, it seems likely that at least some of the chromosomal rearrangements may have had a significant impact on reproductive isolation when they occurred.”
Thus, it isn’t clear (from the ape evidence) that even chromosomal rearrangements would have rendered the different types of Homo infertile, but it is clear that there were fewer such events, which even might have caused reproductive isolation, than there are recognized taxons of Homo. In other words, just because erectus was different enough to be a recognized taxon doesn’t mean they could not interbreed with sapiens.
The cited authors state there have been five or fewer genetic-isolation-speciations since the last common ancestor with chimps:
”From the above evidence we conclude that the number of species on the DLMH, as inferred from human chromosome rearrangements, might be around 3 and cannot be more than 5.”
So all of the types of Homo living in the last few hundred thousand years would have been fertile with the other types. H. sapiens/sapiens and H. erectus and H. neanderthalensis would have all been able to interbreed … and the genetic evidence, as presented in the papers posted on this site, indicates they DID interbreed, resulting in the modern populations.