Oprindelseshistorie:Omskrivning af menneskets historie gennem DNA

I det meste af vores evolutionære historie – i det meste af tiden har anatomisk moderne mennesker været på Jorden – har vi delt planeten med andre arter af mennesker. Det har kun været i de sidste 30, 000 år, blot et blink med et evolutionært øje, at moderne mennesker har besat planeten som den eneste repræsentant for hominin-slægten.

Men vi bærer beviser på disse andre arter med os. Der lurer i vores genom spor af genetisk materiale fra en række gamle mennesker, som ikke længere eksisterer. Disse spor afslører en lang historie med sammenblanding, som vores direkte forfædre mødte – og parrede sig med – arkaiske mennesker. Efterhånden som vi bruger stadig mere komplekse teknologier til at studere disse genetiske forbindelser, vi lærer ikke kun om disse uddøde mennesker, men også om det større billede af, hvordan vi udviklede os som art.

Joshua Akey, en professor ved Lewis-Sigler Institute for Integrative Genomics, står i spidsen for bestræbelserne på at forstå dette større billede. Han kalder sin forskningsmetode for genetisk arkæologi, og det ændrer, hvordan vi lærer om vores fortid. "Vi kan udgrave forskellige typer mennesker, ikke fra snavs og fossiler, men direkte fra DNA, " han sagde.

Ved at kombinere sin ekspertise inden for biologi og darwinistisk evolution med beregningsmæssige og statistiske metoder, Akey studerer de genetiske forbindelser mellem moderne mennesker og to arter af uddøde homininer:Neandertalere, palæoantropologiens klassiske "hulemænd"; og Denisovans, et nyligt opdaget arkaisk menneske. Akeys forskning afslører en kompleks historie om sammenblandingen af ​​tidlige mennesker, tegn på flere årtusinders befolkningsbevægelser over hele kloden.

"Der er ofte et skel mellem de forskere, der går ud og indsamler eksotiske prøver, og de forskere, der laver virkelig kreativ teori og dataanalyse, og han har gjort begge dele, " sagde Kelley Harris, en tidligere kollega til Akey's, som nu er assisterende professor i genomvidenskab ved University of Washington.

Ligesom mange af os, Akey har længe været interesseret i, hvordan den menneskelige art udviklede sig. "Folk vil gerne lære om deres fortid, " sagde han. "Men endnu mere end det, vi vil gerne vide, hvad det vil sige at være menneske."

Denne nysgerrighed fulgte Akey gennem hele hans skolegang. Under sit kandidatarbejde ved University of Texas Health Science Center i Houston i slutningen af ​​1990'erne, han så på, hvordan nutidige mennesker i forskellige dele af verden var genetisk beslægtede med hinanden, og brugte tidlige gensekventeringsmetoder til at forsøge at forstå disse sammenhænge.

Gensequencere er enheder, der bestemmer rækkefølgen af ​​de fire kemiske baser (A, T, C og G), der udgør DNA-molekylet. Ved at bestemme rækkefølgen af ​​disse baser, analytikere kan identificere den genetiske information, der er kodet i en DNA-streng.

Siden 1990'erne, imidlertid, gensekventeringsteknologien har udviklet sig dramatisk. En ny teknologi kendt som næste generations sekventering kom i brug omkring 2010 og gjorde det muligt for forskere at studere et meget stort antal genetiske sekvenser i det menneskelige genom. Det tog 10 år at sekventere det første menneskelige genom, men disse nye maskiner får hele genomsekvensdata fra tusindvis af individer på kun få timer. "Da næste generations sekventeringsteknologi begyndte at blive den dominerende kraft inden for genetik, "Akey sagde, "det ændrede fuldstændigt hele feltet. Det er svært at overvurdere, hvor dramatisk denne teknologi har været."

Omfanget af de data, der nu kan analyseres, har gjort det muligt for forskere at løse en hel række nye spørgsmål, som ikke ville have været muligt med den tidligere teknologi.

One of these questions is the relationship between modern humans and archaic humans, such as Neanderthals. Faktisk, this question fostered a vigorous debate about whether modern humans carried genes from Neanderthals. I mange år, the opinions of researchers—both pro and con—ticked back and forth like a metronome.

Lidt efter lidt, imidlertid, a few researchers—including geneticists Svante Pääbo of the Max Planck Institute in Germany and his colleague Richard (Ed) Green of the University of California-Santa Cruz—began to demonstrate strong evidence that, Ja, there had been gene flow from Neanderthals to modern humans. In a 2010 paper, these researchers estimated that people of non-African ancestry had about 2% Neanderthal ancestry.

Neanderthals lived in a wide geographical swath across Europe, the Near East and Central Asia before dying out around 30, 000 år siden. They lived alongside anatomically modern humans, who evolved in Africa some 200, 000 år siden. The archaeological record shows that Neanderthals were adept at making stone tools and developed a number of physical traits that uniquely adapted them to cold, dark climates, such as broad noses, thick body hair and large eyes.

Following on the heels of Pääbo and Green's Neanderthal research, Akey and a colleague, Benjamin Vernot, published a paper in Science looking at recovering Neanderthal sequences from the genome of modern humans. Geneticist David Reich of Harvard University published a similar paper in Nature, og, together, the two papers provided the first data employing the modern genome to investigate our link with Neanderthals.

Using the genetic variation in contemporary populations to learn about things that happened in the past involves scrutinizing the modern human genome for gene sequences that display traits expected to have been inherited from a different type of human. Akey and his colleagues then take those sequences and compare them to the Neanderthal genome, looking for a match.

Using this technique, Akey has been able to uncover a rich human legacy of genetic interconnections on a scale previously unconceived. As stated, while the available evidence suggests that non-Africans carry about 2% of Neanderthal genes, Africans, who were once believed not to have any connections with Neanderthals, actually have approximately 0.5% Neanderthal genes. Researchers have further discovered that the Neanderthal genome has contributed to several diseases seen in modern human populations, såsom diabetes, arthritis and celiac disease. By the same token, some genes inherited from Neanderthals have proven beneficial or neutral, such as genes for hair and skin color, sleep patterns and even mood.

Akey has also discovered genetic fingerprints that suggest our human ancestry contains species about which we know nothing or very little. The Denisovans are a case in point. An archaic form of human, they coexisted with anatomically modern humans and Neanderthals and interbred with both before going extinct. The first evidence of their existence came in 2008 when a finger bone was discovered in Denisova Cave in the remote Altai Mountains of southern Siberia. At first the bone was assumed to be Neanderthal because the cave contained evidence of these species. Følgelig, it sat in a museum drawer in Leipzig, Tyskland, for many years before it was analyzed. But when it was, the researchers were dumbfounded. It wasn't a Neanderthal—it was a hitherto unknown type of ancient human. "The Denisovans are the first species ever identified directly from their DNA and not from fossil data, " Akey said.

Since that time, continued genetic work—much of it conducted by Akey and his colleagues—has established that the closest living relatives of Denisovans are modern Melanesians, the inhabitants of the Melanesian islands of the western Pacific—places such as New Guinea, Vanuatu, the Solomon Islands and Fiji. These populations carry between 4% and 6% of Denisovan genes, though they also carry Neanderthal genes.

Examples like this highlight one of the main features of our human lineage, Akey said, that admixture has been a defining feature of our history. "Throughout human history there's always been admixture, " Akey said. "Populations split and they come back together."

While there remains a lot of debate about the Denisovans, Akey believes they most likely were closely related to Neanderthals, perhaps an eastern version who split off from the latter sometime around 300, 000 or 400, 000 år siden. For nylig, genetic analysis of fossils from Denisova Cave has uncovered evidence of an offspring between a Neanderthal woman and a Denisovan male. The offspring was a female who lived approximately 90, 000 år siden. By looking at this genetic trail, Akey and other researchers have been able to piece together a fascinating story of human evolution—one that is promising to rewrite our understanding of early human origins.

But there's so much more to discover, Akey said. "Even though we have sequenced probably 100, 000 genomes already, and we have pretty sophisticated tools for looking at that variation, the more we think about how to interpret genetic variation, the more we find these hidden stories in our DNA, " han sagde.