Neandertals and Early Humans:
But Did They Mate?
Linda Strausbaugh and Sally Sakelaris
The University of Connecticut
NABT Convention, Montreal
7 November 2001
|Synopsis: Using mitochondrial DNA, sampled from 3 different Neandertal fossils, widely separated in time and geography, independently corroborated conclusion that Neandertals probably did NOT contribute to modern humans. Note the variety of educational applications of these studies in the first paragraph.|
DNA and Early Human History: Neandertals and Early Humans
... But Did They Mate?
Studies in Molecular Evolution, especially those with forensic,
ancient DNA or human history implications, provide a compelling
framework for teaching. Since such research topics tend to capture
the attention and imaginations of students, they provide excellent
opportunities for instruction in basic biological disciplines
such as evolutionary biology,
Neandertals, named after the German valley (Neander Tal) where
their fossil remains were first discovered in 1856, are extinct
hominins (formerly "hominids") that lived in Europe
and Western Asia. Living from approximately 30,000 to 300,000
years ago, they were the prototype for the "robust"
classic "caveman", in contrast to the "gracile"
modern human. Neandertal skeletons suggest they were a larger
and more muscular version of modern man with low foreheads, protruding
brows, poorly defined chins, and large noses with broad nostrils.
Portrayed at various times in modern history as brutish, clumsy,
Archaeological evidence reveals that the Neandertals disappeared
somewhat abruptly 25,000 to 30,000 years ago. Their demise was
preceded by the arrival in their geography of our direct ancestor,
the anatomically modern Cro-Magnons. There is ample rchaeological
evidence that the two groups likely co-existed and even sequentially
inhabited the same sites over perhaps thousands of years. Were
the two groups completely unaware of each other, and thus could
not interact at all? Were the two aware of each other only through
distant observation? Or as some scientists believe, did they
Another way of stating this is if there were widespread, successful matings among Neandertals and the ancestors of modern humans, then there would be contribution of Neandertal genetic sequences to the genomes of modern humans.
Three hypotheses about the nature of the genetic relationship
between Neandertals and modern humans have been offered. At one
extreme is a replacement hypothesis, proposing that the Neandertals
were a fundamentally different type of human (perhaps even a
different species) that represent an evolutionary dead-end, with
no genetic contributions to present-day humans. At the other
extreme is a linear evolution hypothesis that modern humans in
Europe evolved directly from Neandertals, providing major genetic
contributions to present-day humans. In an intermediate model,
Testing these hypotheses requires a comparison of DNA from
Neandertal and modern human samples - a formidable task at best.
Technical challenges abound in the analysis of ancient DNA. Accurately
dated, well-preserved fossils are rare and extremely valuable.
Even when such exist, the DNA will be subjected to normal degradative
processes that occur over time, resulting in both very short
segments of DNA and alterations in chemical structure. The extremely
sensitive polymerase chain reaction (PCR) that is instrumental
in retrieving DNA segments from ancient DNA is particularly prone
to problems with damaged DNA. Further complicating the problem
is the likelihood that handling of the fossils by modern humans
may have resulted in contamination. For these reasons, a number
of criteria for acceptance of ancient DNA
Nuclear genes occur in two copies per cell. The combination
of their low abundance and the aforementioned technical problems
make it highly unlikely that nuclear DNA sequences will be amenable
to analysis in ancient samples. Scientists have concentrated
instead on the use of mitochondrial DNA (mt DNA). The mitochondrial
genome is present in hundreds to thousands of copies in each
cell, providing an important technical advantage. The control
region of the mitochondrial genome does not encode proteins,
and contains hypervariable regions that are known to reveal differences
In what has been widely hailed as a milestone in genetic studies
of early human history, Svante Paabo and his collaborators (Krings
et al., 1997) successfully isolated and analyzed a segment of
Neandertal DNA. They used PCR to amplify sequences from the control
region in mitochondrial DNA from samples isolated from a piece
of right humerus from the Neandertal type specimen. They obtained
the Neandertal sequence of one of the hypervariable regions and
compared it to the reference sequence for modern humans. The
Neandertal sequence was closer to that of humans than to chimpanzees,
although there were dramatic differences from modern humans.
While acknowledging this research as a pioneering study in
Neandertal scholarship, the scientific community (including the
authors) raised the problems inherent in generalizing from a
single sample. What if this individual were at an extreme in
the genetic variation present in the Neandertal population? Could
there be other, more prevalent, Neandertal
For the conclusion that Neandertals do not contribute to modern
gene pools to be correct, it is necessary for the known human
sequences to be an accurate representation of the event in question.
The unique inheritance patterns and evolutionary pressures on
mtDNA make it reasonable to question whether it accurately represents
all possible genetic athways. Since we do not yet have (and may
never get) other DNA sequences from ancient DNA, this is something
of a moot objection. Assuming that mtDNA is the only option,
it is important to note that the current human database includes
over one thousand
A more thorny issue is whether the mtDNA variation present
in modern people is an accurate representation of our full genetic
history. What if the existing variation is but a subset of that
which has occurred in the direct lineage of modern man? Is it
possible that Neandertals mated extensively with our ancestors
and that the Neandertal type mtDNA contribution has been lost?
In other words, could an absence of Neandertal mtDNA types in
modern humans be due to forces of evolution other than reproductive
Recently, Adcock et al. (2001) provided evidence that mtDNA sequences can "go extinct". Among the remains of ten ancient Australian individuals typed for mtDNA sequences was an anatomically modern man from Lake Mongo. The remains of the Lake Mongo man had been dated by three separate methods to be older than 60,000 years. The Lake Mongo man mtDNA contains a sequence that is different from the other fossil remains and different from all modern humans, meaning that he must have possessed a now extinct lineage of mtDNA. Interestingly, a remnant of this particular mtDNA sequence does survive in modern people as an inserted segment on chromosome 11 in the nucleus! If a mtDNA sequence found in an early modern human can so easily "go extinct", the possibility exists that the same thing could have happened with a mtDNA from Neandertals.
While it remains to be seen whether the interactions between Neandertals and our ancestors were G, R, or X-rated, it is certain that the Neandertals and Cro-Magnons did not interact as portrayed in "B" movies. Neandertals were hardly the simple-minded, big and bumbling brutes who fell prey to the invading, intelligent and gracile Cro-Magnons. Both types of "humans" walked the earth together for thousands of years in and around Europe. Like many of the most interesting evolutionary questions, the issues surrounding the co-existence of Neandertal (and other ancient hominins) with our early ancestors will be investigated for years to come as new specimens and new approaches come to light. The final chapter in the relationship between Neandertal and modern man remains to be written..
MINIREVIEWS & NEWS
"DNA From an Extinct Human" by Patricia Kahn and Ann Gibbons. Research News. SCIENCE 277:176-178. 11 July 1997.
"Facts and Artifacts of Ancient DNA" by Tomas Lindahl.
"Ancient DNA: Neanderthal population genetics" by
Matthias Hoss. News and
"The Riddle of Coexistence" by Ann Gibbons. News Human Evolution:Migrations. Includes sidebars "But Did They Mate?" by Anne Gibbons and "Anthropologists Duel Over Modern Human Origins" by Michael Balter. SCIENCE 291:1725-1729. 2 March 2001.
"Oldest Human DNA Reveals Aussie Oddity" by Constance Holden and "Skull Study Targets Africa-Only Origins" by Elizabeth Pennish. Research News. SCIENCE 291:230-231. 12 Jan. 2001.
"What - or Who - Did In the Neandertals?" by Michael
Balter. Research News.
ORIGINAL LITERATURE ARTICLES
"Neandertal DNA Sequences and the Origin of Modern Humans". Matthias Krings, Anne Stone, Ralf Schmitz, Heike Krainitzki, Mark Stoneking, and Svante Paabo. Cell 90:19-30. July 11, 1997.
"Molecular analysis of Neanderthal DNA from the northern
Caucasus". Igor Ovchinnikov, Anders Gotherstrom, Galina
Romanova, Vitaly Kharitonov, Kerstin
"A view of Neandertal genetic diversity". Krings, M., Capellini, C., Tschentshcer, F., Geisert, F., Meyer, S., von Haeseler, A., Grossschmidt, K., Possnert, G., Paunovic, M. and Paabo, S. Nature Genetics 26:144-146. 2000
"Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins". Gregory Adcock, Elizabeth Dennis, Simon Easteal, Gavin Huttley, Lars Jermiin, W. James Peacock, and Alan Thorne. Proc. Natl. Acad. Sci. 98: 537-542. January 16, 2001.