Παρασκευή, 24 Δεκεμβρίου 2010

Ντενίσοβαν: Νέο είδος ανθρώπου ανακαλύφθηκε στη Σιβηρία / Denisovans: A New Type of Ancient Human found in Siberia


Αυτό το ενήλικο δόντι αποτελεί ένα από τα δύο γνωστά φυσικά κατάλοιπα των Ντενίσοβαν
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This adult tooth is one of only two known physical remains of the Denisovan humans
Photo by David Reich, ''Nature''

Ένα νέο είδος ανθρώπου που ζούσε πριν από 30.000 χρόνια, παράλληλα με τους προγόνους μας ανακαλύφθηκε από επιστήμονες στη Σιβηρία.

Τα ευρήματα δηλώνουν ότι εκείνη την εποχή υπήρχαν εν ζωή τουλάχιστον τρία μέλη της οικογένειας των ανθρώπων. Οι σπηλαιάνθρωποι, που ονομάστηκαν Ντενίσοβαν (Denisovan), αναγνωρίστηκαν από το DNA που βρέθηκε σε ένα δόντι και ένα κόκαλο δαχτύλου.

Σύμφωνα με τα στοιχεία, αυτό το είδος ανθρώπου φαίνεται από είναι το τρίτο γνωστό, μετά τους Νεάντερνταλ (Homo Neanderthalensis) και τον σύγχρονο άνθρωπο (Homo Sapiens Neanderthalensis) και περπάτησε στη Γη κατά τη διάρκεια της τελευταίας εποχής των Παγετώνων, όταν οι σύγχρονοι άνθρωποι ανέπτυσσαν εκλεπτυσμένα πέτρινα εργαλεία, κοσμήματα και τεχνουργήματα.

Το γενετικό υλικό που βρέθηκε ανήκει σε ένα νεαρό κορίτσι ενός εντελώς νέου ανθρώπινου είδους, και έχει ονομαστεί γυναίκα-Χ. Το κορίτσι, που πρέπει να ήταν μεταξύ πέντε και επτά ετών όταν πέθανε, βρέθηκε σε σπηλιά στα βουνά Αλτάι.

Παρόλα αυτά οι μελετητές έχουν βρει ότι DNA των Ντενίσοβαν υπάρχει στους σύγχρονους της Μελανησίας, δηλαδή του νησιωτικού συμπλέγματος που βρίσκεται στα βορειοανατολικά της Αυστραλίας και περιλαμβάνει τη Νέα Γουινέα. Αυτό υποδηλώνει ότι οι Ντενίσοβαν αναμείχθηκαν με τους προγόνους των Μελανησίων και ότι ενδέχεται να εξαπλωμένοι στην Ασία.


English version
A previously unknown kind of human, the Denisovans, likely roamed Asia for thousands of years, probably interbreeding occasionally with humans like you and me, according to a new genetic study.

In fact, living Pacific islanders in Papua New Guinea may be distant descendants of these prehistoric pairings, according to new analysis of DNA from a girl's 40,000-year-old pinkie bone, found in Siberian Russia's Denisova cave.

This "new twist" in human evolution adds substantial new evidence that different types of humans—so-called modern humans and Neanderthals, modern humans and Denisovans, and perhaps even Denisovans and Neanderthals—mated and bore offspring, experts say.

"We don't think the Denisovans went to Papua New Guinea", located at the northwestern edge of the Pacific region called Melanesia, explained study co-author Bence Viola, an anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

"We think the Denisovan population inhabited most of eastern Eurasia in the same way that Neanderthals inhabited most of western Eurasia. Our idea is that the ancestors of Melanesians met the Denisovans in Southeast Asia and interbred, and the ancestors of Melanesians then moved on to Papua New Guinea", Viola said.

Fossil Finger Points to New Human Type
The centerpiece of the DNA study is a Denisovan fossil finger bone discovered in 2008. The fossil is thought to be from a young girl—dubbed X-woman—who was between 5 and 7 years old when she died.

For a previous ''Nature'' study, released in March 2010, the team had collected and sequenced mitochondrial DNA, or mtDNA, from X-woman's finger. But mtDNA—inherited only from mothers—contains far less information about a person's genetic makeup than DNA found in the nucleus of a cell, or nuclear DNA.

In the new study the team reports successfully extracting and sequencing nuclear DNA from the bone. Then, using DNA-comparison techniques, the scientists were able to determine that Denisovans were distinct from both modern humans and Neanderthals, yet closely related to the latter. The team estimates Denisovans split from the parent group of Neanderthals about 350,000 years ago.

New Humans Had Huge Teeth
Along with the finger bone, archaeologists from the Russian Academy of Sciences, who excavated the site, discovered a single tooth that belonged to a Denisovan adult.

The tooth, a molar, is bigger than any modern human tooth and is even bigger than the biggest Neanderthal tooth. This could suggest Denisovans were "comparable in size to Neanderthals, maybe a little bit bigger", said George Washington University's Richmond.
 
Richmond cautioned, however, that tooth size isn't always a good indicator of body size. A hominin "can have big teeth and not be a giant", he said.

Denisovans a New Human Species?
The team has been careful not to call Denisovans a new species, opting instead to label them as a Neanderthal "sister group".

If modern humans and Denisovan humans were separate species, their hybrid children probably wouldn't have been able to reproduce. But the hybrids apparently were able to have babies, otherwise the Denisovan DNA couldn't have been passed down to today's Papua New Guineans. Therefore, study co-author Viola reasoned, Denisovans and modern humans probably weren't separate species.

Given the mounting evidence that modern humans interbred with Neanderthals and now Denisovans, some evolutionary biologists have even suggested dropping the species designation for Neanderthals and modern humans.

As scientists "produce evidence that Denisovans interbred with modern humans (as did Neanderthals) then the implication is that modern humans, Denisovans and Neanderthals are all subspecies of Homo sapiens", he said.

It's indisputable, though, that each of these groups was genetically distinct, said George Washington University's Richmond. "Whether you call them subspecies or species, it is clear that modern humans, Neanderthals, and now Denisovans were separated for hundreds of thousands of years, and only later did some of them meet and interbreed".

Πηγές / Sources: To Vima, National Geographic

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