Why didn't our branch of humanity interbreed with Neandertals?

Question

Our variety of human beings (homo sapiens sapiens) and that other bunch (homo sapiens neandertalis) were both around 30,00 years ago, about the time that the "iceman" got frozen in the Alps. The present human genome varies little from that of the "iceman", showing that we never did breed with our probably equally-intelligent cousins, whose greatest cultural difference seems to be that they confined sex to a rutting season whereas it has always been "open season" with us. Were we mutually "ugly" to each other, or did breeding produce sterile offspring. (I can't imagine that there weren't curious people in both groups who experimented with it!)

Answer 1

Homo sapiens didn't interbreed with Neanderthals because they were a separate species. Genetic evidence, amongst other findings, has shown that Neanderthals were not the same species as us.

Genetic analysis of Y-Chromosomal DNA supports the separate species view as well as shown in a presentation by Svante Paabo (as reported by Nature News):

"One finding so far is that the Neanderthal Y chromosome is substantially more different from human and chimp Y chromosomes than are other chromosomes. This suggests that little interbreeding occurred, at least among the more recent Neanderthal species."

Source - Dalton, R. (2006). Neanderthal DNA yields to genome foray. Nature, 441, 260-261.

The report was given at New York’s Cold Spring Harbor Laboratory.

Back in 1997 Matthias Krings and his team extracted mtDNA from a Neanderthal specimen found in Germany. Using PCR techniques and control groups for comparison, Krings found that when compared to modern humans, Neanderthals show differentiation to the extent of being a separate species:

"DNA was extracted form the Neanderthal-type specimen found in 1856 in western Germany. By sequencing clones from short overlapping PCR products, a hitherto unknown mitochondrial (mt) DNA sequence was determined. Multiple controls indicate that this sequence is endogenous to the fossil. Sequence comparisons with human mtDNA sequences, as well as phylogenetic analyses, show that the Neanderthal sequence falls outside the variation of modern humans."

Krings, M., Stone, A., Schmitz, R., Krainitzki, H., Stoneking, M. and Paabo, S. (1997). Neanderthal DNA sequences and the origin of modern humans. Cell, 90, 19-30.

Genetic analysis done by Scholz et al. (2000) showed that Neanderthals are not the ancestors of modern humans and not were differentiated enough to show they are not anatomically modern humans.

Using PCR amplification on two different samples of Neanderthal DNA (one from Germany and one from Croatia), they were compared to Homo sapien DNA from a German sample and mammoth and reindeer DNA for controls (from Banks Island, Canada).

"The data presented in this study indicate that the composition of the genomes of Neanderthals and anatomically modern man is significantly different…(1930)"

Scholz continues to examine more in depth the genetics and concludes:

"…the mtDNA sequences provided by Krings et al. (1997, 1999) and the data presented here indicate a clear differentiation of Neanderthals and anatomically modern man and support the assumption that both taxa are discrete species. This also means Neanderthals cannot be the direct ancestor of anatomically modern humans. (1931)"

Source – Scholz, M., Bachmann, L., Nicholson, G., Bachmann, J., Giddings, I., Ruschoff-Thale, B. et al. (2000). Genomic differentiation of Neanderthals and anatomically modern man allows a fossil-DNA-based classification of morphologically indistinguishable hominid bones. American Journal of Human Genetics, 66, 1927-1932.

A unique study into the Neanderthal/modern human differentiation comes from an examination done by Grine and molar enamel. In examining the enamel thickness and molar morphology, Grine found:

"Evaluation of this purported synapomorphy through an examination of the permanent molars of recent Europeans and Africans reveals that they are indistinguishable from one another in enamel thickness. Thus, modern Europeans do not share relatively thin molar enamel with Neanderthals. Rather, thin tooth enamel would appear to represent an ostensibly autapomorophic feature by which they differ from modern as well as other fossil humans. (389)"

Source – Grine, F. (2004). Geographic variation in human tooth enamel thickness does not support Neanderthal involvement in the ancestry of modern Europeans. South African Journal of Science, 100, 389-394.

If Neanderthals and modern humans were the same species they would have been able to admix. Currat and Excoffier’s research shows this did not happen during the modern humans expansion into Europe.

"…we estimate that maximum interbreeding rates between the two populations should have been smaller than 0.1%. We indeed show that the absence of Neanderthal mtDNA sequences in Europe is compatible with at most 120 admixture events between the two populations despite a likely cohabitation time of more than 12,000 y. This extremely low number strongly suggests an almost complete sterility between Neanderthal females and modern human males, implying that the two populations were probably distinct biological species. (2264)"

Source – Currat, M. and Excoffier, L. (2004). Modern humans did not admix with Neanderthals during their range expansion into Europe. PLoS Biology, 2, 2264-2274.

Currat and Excoffier’s work supports similar work previously done four years earlier by Ovchinnikov et al. in 2000.

Ovchinnikov and his team found that the genetic evidence shows that Neanderthals and humans are two separate species and Neanderthals did not contribute to the modern human gene pool. The examined a specimen from Mezmaiskaya Cave in northern Caucasus which was 29,000 years old.:

"The sequence shows 3.48% divergence from the Feldhofer Neanderthal. Phylogenetic analysis places the two Neanderthals [referring to a previous study by Ovchinnikov on a different specimen] from the Caucasus and western Germany together in a clade that is distinct from modern humans, suggesting that their mtDNA types have not contributed to the modern human mtDNA pool. Comparison with modern populations provides no evidence for the multiregional hypothesis of modern human evolution. (490)"

Source – Ovchinnikov, I., Gotherstrom, A., Romanova, G., Kharitonov, V. Liden, K. and Goodwin, W. (2000). Molecular analysis of Neanderthal DNA from the northern Caucasus. Nature, 404, 490-493.

Harvati, Frost and McNulty focused on morphological differences between Neanderthals, modern humans, gorillas and chimps and some old world monkeys. Their analysis found:

"Morphological distances between model taxon pairs were compared to the distances between Neanderthals and modern humans obtained by using a randomization technique. Results strongly support a specific distinction for Neanderthals. (1147)"

Source – Harvati, K., Frost, S. and McNulty, K. (2004). Neanderthal taxonomy reconsidered: Implications of 3D primate models of intra- and interspecific differences. Nature, 101, 1147-1152.

Bruner et al. focused on a comparison of endocasts of Neanderthal and modern humans. One of the major events in our lineage’s evolution was the encephalization of the brain. The comparison of modern humans and Neanderthals showed marked differences:

"Enodcranial morphology was studied in a sample of fossil hominines by multivariate approaches using both traditional metrics and geometric morphometrics.

The main result was the identification of two different evolutionary trajectories, in which a similar expansion in endocranial size has been reach by different changes in shape. (15335)"

Source – Bruner, E., Manzi, G. and Arsuaga, L. (2003). Encephalization and allometric trajectories in the genus Homo: Evidence from the Neanderthal and modern lineages. PNAS, 100, 15335-15340.

Caramelli et al. studied two Cro-magnon specimens and compared them with Neanderthal samples of DNA. They found what many others have and keep finding, more evidence that Neanderthals and modern humans are two distinct species:

"Following the most stringent current standards for validation of ancient DNA sequences, we typed the mtDNA hypervariable region I of two anatomically modern Homo sapiens sapiens individuals of the Cro-Magnon type dated at about 23 and 25 thousand years ago. Here we show that the mtDNAs of these individuals fall well within the range of variation of today’s humans, but differ sharply from the available sequences of the chronologically closer Neanderthals. (6593)"

Source – Caramelli, D., Lalueza-Fox, C., Vernesi, C., Lari, M., Casoli, A., Mallegni, F. et al. (2003). Evidence for a genetic discontinuity between Neanderthals and 24,000-year-old anatomically modern Europeans. PNAS, 100, 6593-6597.

Tattersall accentuates the findings of the status of Neanderthals being a separate species when discussing the cohabitation of modern humans and Neanderthals:

"…if the Neanderthals were a separate species from us – which the continuity people would deny, of course – significant interchange of genes would have not been possible (though, possibly, individuals might willingly or unwillingly have participated in attempts to hybridize). (226)"

The non-mixing of genetics has been shown and supported and therefore supports Tattersall’s contention that they were, indeed, a separate species. In fact, Tattersall was on of the first people to formally propose the separate species distinction which was originally determined in the early part of the 20th century:

"Specifically I urged that, at the very least, the Neanderthals be restored to separate species status as Homo neanderthalensis. (219)"

Tattersall, I. (1995). The Fossil Trail. New York: Oxford U.P.

Kottak sums up the contention over the admixture vs. replacement hypothesis (replacement including the non-contribution of Neanderthals to the human gene pool which is supported many times over by genetic evidence):

"Current interpretations of the fossil evidence and dating seem to support the replacement hypothesis, which denies the Neanderthal ancestry of AMHs [anatomically modern humans] in Western Europe and the Middle East. AMHs seem likely to have evolved from an archaic H. sapiens African ancestor [Homo heidelbergensis]. In Africa, as in the Middle East and Asia, the archaic H. sapiens fossils generally had flatter, less projecting faces than the Neanderthals did. Eventually, AMHs spread to other areas, including Western Europe…(175)"

Kottak, C. (2002). Anthropology. (9th ed.). New York: McGraw-Hill.

The list which I posted earlier showed many other differences between modern humans and Neanderthals in behavior and ecology from paleoanthropological evidence. As Klein notes:

"…archaeology also suggests important behavioral differences. Unlike Upper Paleolithic Cro-Magnons, Middle Paleolithic Neanderthals left little compelling evidence for art or jewelry.

Their cave sites are generally poorer in cultural debris and richer in bones of bears and other cave dwellers.

Finally, the Paleolithic artifact assemblages that Neanderthals produced varied little through time and space. The Upper Paleolithic assemblages that Cro-Magnons made varied far more and are the oldest from which we can infer identity-conscious ethnic groups."

Klein, R. (2003). Whither the Neanderthals? Science, 299, 1525-1528.

Poirier and McKee discuss more morphological aspects of Neanderthals as they differ from modern humans:

"Face: Overall facial skeleton is massive; midfacial prognathism, large nasal apertures, well-developed but discontinuous brow ridges.

Skull: The cranial vault is long, low, and wide. The occipital bones have occipital ridges, and many have occipital buns. Perhaps differently shaped foramen magnum and middle ear bones.

Dentition and Jaws: No canine diastema in maxillary region; molars have large pulp cavity (taurodontism); large incisors, no chin.

Spinal Column: Heavily built; the cervical vertebrae have long projecting spinous processes.

Upper Limb Skeleton: Broad scapula, robust humerus with massive head.

Pelvis: More dorsally rotated ilia than in modern humans.

Lower Limb Skeleton: Massive femurs, short and strong tibiae, large and thick kneecaps. (271)"

Poirier, F. and McKee, J. (1999). Understanding Human Evolution. (4th ed.). Upper Saddle River: Prentice Hall.

Answer 2

It may well be that we did indeed interbreed. I heard once that one theory has it that red hair in humans comes from Neanderthal genes. I have also heard that three living Neanderthals were found in an isolated tribe in Morocco sometime over 100 years ago. If we lived together, we interbred. Case in point, Down's Syndrome. They are unattractive, and very different, yet alike enough that sex between Down's and non Downs is not uncommon....see this week's Time Magazine.

Answer 3

Genetics. We may have looked similar but we couldnt interbreed. Neanderthals were from a different family line and looked like us and had similar traits due to convergant evolution.
It has been found to be Homo neanderthalensis
Not a subspecies of sapiens and thanks to mitochondrial DNA studies it has been proven that modern humans contain no Neanderthal DNA

Answer 4

How come people can't condense an article and then paste the url instead of pasting 25 pages. ??? Huh !

And the answer to your question is, YES they did breed !
Look at George Bush and tell me they didn't !!!!!!!!!!

Answer 5

great question! but scientists dont know either!