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Thursday, April 22, 2010

homo incognitus

By Cinthia Briseño


Have scientists identified a "homo incognitus" -- a previously unknown
human species? Finger bones dating from 30,000 years ago were
unearthed in southern Siberia. Its genes differ from those of modern
humans as well as Neanderthals, and German scientists think they are
onto a sensation.


John Krause checked his findings again and again. Somehow he couldn't
believe what the analysis was showing. The scientist wanted to make
sure he was right before phoning his boss, the renowned evolutionary
genetics specialist Svante Pääbo. Did the DNA really stem from a
previously unknown human form?


The Max Planck Institute for Evolutionary Anthropology in Leipzig had
just 30 milligrams of bone powder available to carry out its genetic
analysis. From that sample, Krause and his colleagues isolated the DNA
of a primitive human who had lived anywhere between 30,000 and 48,000
years ago in the Denisova Cave in the Altai Mountains of southern
Siberia. The scientist began with low expectations, seeing the job as
a "routine investigation of an ancient fossil," Krause told SPIEGEL
ONLINE. "That piece of bone was sold to us as a specimen from early
modern humans." Russian scientists had unearthed the fossil fragment
in 2008.


Isolating DNA from fossil bones is no easy task, not least because the
ancient pieces of bone also contain DNA from other organisms such as
bacteria or fungi, possibly even from other early humans. If
laboratory conditions fail to meet the strictest standards, there is
the risk of contaminating the sample with the genetic material of a
present-day scientist.


So Krause had to rule out these factors. But eventually he believed
the genetic material was several thousand years old, harbored intact
within the cells of a small chunk of bone, no bigger than a cherry
pip. Whichever way he examined the find, the DNA sequence did not
resemble anything any anthropologist had ever seen before. Had the Max
Planck researchers discovered a new species of early man? Did it
belong to an unknown hominin, whose DNA differed so much from
Neanderthal and early modern humans that it could revolutionize our
understanding of the evolution of man?


Bred in the Bone


The genetic material found in the Denisova Cave is a complete sequence
of so-called mitochondrial DNA, the genetic material found in a cell's
mitochondria. Within these organelles, the cell's power plants,
biochemical processes take place which enable the survival of a whole
organism. Each mitochondrion has its own genome, which is passed from
mothers to offspring.


And each cell has approximately 8,000 copies of mitochondrial DNA. But
the mitochondrial genome is far smaller than the nuclear genome. Only
around 16,500 building blocks (or base pairs) sit side-by-side in the
mitochondrial DNA, compared to approximately three billion in the DNA
of the nucleus.


Every base pair of the mitochondrial DNA was sequenced by Krause over
and over again, up to an average of 156 times each. Afterwards
researchers compared the mitochondrial DNA sequence with others --
including 54 sequences belonging to modern man, one belonging to a
recently discovered early human from Kotenski in Russia, six from
Neanderthals, and one each from a bonobo and a chimpanzee.


The mitochondrial DNA of Neanderthals differs on average in 202 ways
from the mitochondrial DNA of modern humans. There were twice as many
differences separating the Denisova Cave individual from modern man.


Amazing Results


"The results practically blew me away when I heard," says Svante
Pääbo, director of the genetics department at the Max Planck
Institute, who received the phone call while he was in the US. Once
Pääbo had convinced himself of the findings, he too was certain that
the science of human evolution was about to change.


So how should we understand this stranger from southern Siberia, if
not as an early modern man or as a Neanderthal? Researchers still
don't know the hominin's gender. To find out, Pääbos' research team
needs to examine the nuclear genome, one base pair at a time. Only
then can they make more precise statements about the skin and eye
color, blood type, or the health of the specimen. These investigations
are painstaking but not impossible: A Danish research team recently
gleaned similar information from a 4,000-year-old hair belonging to a
Paleolithic eskimo.


For years, anthropologists believed only two species of the genus Homo
lived on earth around 40,000 years ago. The first group, Neanderthals,
lived in large areas of Europe and northern Asia, but disappeared some
25,000 years ago for reasons which are not completely clear. The other
group, anatomically modern humans -- our direct ancestors -- scattered
across Eurasia after leaving the African continent.


Until now researchers have also assumed at least three waves of
emigration from Africa, one of which took place about 1.9 million
years ago, when an earlier group of hominins, Homo erectus, left the
continent. While there are fossils to prove that Homo erectus may have
survived in Indonesia until up to 100,000 years ago, there is no
evidence that the species might have existed on the Asian mainland as
recently as 30,000 or 40,000 years ago -- the era of the Denisova
finger.


A New Human Family Tree


The genetic analysis that Krause and his colleagues recently published
in the journal Nature adds a new piece to the puzzle of huan
evolution. Once the scientists combined the new data with the
previously known facts they were able to sketch out a new human family
tree. It shows the evolutionary relationship between early modern
humans, Neanderthals and the previously unknown Denisova hominids.


There seems to be only one way to explain the discovery of this
previously unknown strand of DNA: Denisova hominins and the ancestors
of the modern human as well as the Neanderthals must have shared a
common ancestor about one million years ago. That ancestor is about
twice as old as the known common ancestor of anatomically modern
humans and Neanderthals (Homo heidelbergensis).


Moreover, the researchers conclude from the age of the mitochondrial
genome that this previously unknown form of human must have lived
alongside Neanderthals and modern humans.


The researchers are absolutely certain that they are dealing with a
hitherto unknown mitochondrial genome which strongly suggests a new
form of hominin. This would also suggest a fourth wave of emigration
from Africa.


After the release of these sensational results, the Max Planck
researchers will be forced to respond to the same questions they
encountered when they discovered the first version of the Neanderthal
genome: How did this unknown hominin live? Why did they disappear?
And: Did Neanderthals and modern humans fornicate with this Siberian
stranger?


'X-Woman'


To get answers to these questions one will have to wait until the
scientists from Leipzig have analyzed the stranger's entire genome. It
would help the researchers if further fossilized bones of this kind
were found, which would enable study of the hominin's physique.


Krause and Pääbo, speaking at a press conference Tuesday, said that
any further statements at this point would amount to pure speculation.
The researchers won't even announce their intention to present a new
name for this hominin as they can't yet be 100% sure it represents a
whole new species. "The terminology of human species is rather
tricky," Pääbo says.


Internally, the researchers refer to the new hominin form as "X-
Woman." The "X" stands for the unknown factor, and "woman" merely
represents the fact that mitochondrial DNA is inherited from the
mother. Pääbo restrains himself: "Whether or not we academics speak of
a new species and give it a new name is just a matter of pride,
nothing else."

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