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Interbreeding Between Species

*/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 is in agreement 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 int*RA*-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 int*ER*-fertile, just as the genus
Canis is.

Clifford Jolly, writing in the */American Journal of Physical
Anthropology/* (2001; Supplement 33: 177-204) discusses the 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 rearrangement,
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.” 

And concluding,

”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.

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