The person found at Oase, who lived around 40,000 years ago, left no genetic traces in today’s Europeans, but the Markina Gora man, who lived after the volcanic eruption around 38,000 years ago, did. The following scenario therefore seems realistic: the eruption could have decimated or even completely annihilated all the groups of modern humans who came to Europe beforehand—then our direct ancestors, the Aurignacians, arrived in a fresh wave of migration via the Danube corridor.
Even human intelligence is probably a direct consequence of walking on two legs, because the shift to consuming animal fat and protein made it possible for humans to develop an organ that consumes vast amounts of energy. In modern humans the brain demands about a quarter of the body’s energy, although it usually makes up less than 2 percent of a person’s bodyweight.
Walking upright made it possible for Homo erectus to employ a totally new hunting strategy—one that required further mutations, including a gradual loss of hair. Homo erectus could now cover almost limitless distances without overheating, becoming champion endurance athletes.
A gazelle can run fast, but not for prolonged periods, and the same is true for most mammals. They collapse after relatively short distances: horses can gallop for forty kilometers at most. Early humans would simply pursue their prey until the animal was unable to continue.
Neanderthals were living during the Ice Age, hundreds of thousands of years in which vast and insurmountable glaciers periodically formed. Many regions settled by Neanderthals were cut off from the outside world, so they interbred with relatives and harmful mutations spread.
And the Denisovans were even worse off than the Neanderthals. Their DNA shows signs of extensive inbreeding. The ancestors of the Denisova girl were closely related many times over, because large parts of Asia were also sealed off during the Ice Age.
Europeans are 97 to 98 percent descended from Africans and 2 to 2.5 percent descended from Neanderthals. The indigenous populations of Australia and Papua New Guinea are about 7 percent descended from Neanderthals and Denisovans and about 93 percent from Africans. Only the inhabitants of sub-Saharan Africa did not intermix with any other type of archaic human outside the continent.
Roughly 1.9 million years ago, Homo erectus emerged. Within a few hundred thousand years, this hominin would spread throughout the continent and across large parts of Eurasia, becoming the first archaic hominin to leave Africa. In Eurasia, Homo erectus evolved still further, including at one stage into the so-called Peking Man, but then died out. In Africa, meanwhile, the line that led to Neanderthals, Denisovans, and modern humans evolved from Homo erectus at least 600,000 years ago.
An early Neanderthal in Europe or the Near East had mated with this Sapien woman, resulting in a closer relationship between late Neanderthals and modern humans. The Denisovans in Asia, however, did not mix and thus preserved a relatively close resemblance to early Neanderthals in their nuclear and mtDNA.
Everybody has two parents, four grandparents, eight great-grandparents, and sixteen great-great-grandparents. This spans four generations, approximately 80 to 100 years. If we go back twenty generations, 400 to 500 years, we find more than a million ancestors. In the forty generations (at least) that have passed since Charlemagne, we’re looking at more than a trillion. This is admittedly a purely theoretical figure: not everyone had children, and some had more than this calculation takes into account.
If you follow a family tree back in time, you find that many of the lines cross, concentrating around the ancestors who had an above-average number of children. It follows that all the people who had children 600 to 700 years ago and whose descendants continued to produce offspring up to the present day can very likely be found somewhere in the family trees of all living Europeans.
If we compare the mtDNA of two individuals, we can figure out when their most recent matrilineal common ancestor lived—using the molecular clock. The mtDNA of all living modern human beings can be traced back to a single female ancestor, a prehistoric mother. She lived around 160,000 years ago, and in the genetics literature she’s referred to as “mitochondrial Eve.” She has a male counterpart, “Y-chromosomal Adam,” to whom the Y chromosomes passed from father to son can be traced. This Adam lived almost 200,000 years before mitochondrial Eve, however, so we can say with certainty that they weren’t a couple.
Even if you tested the two most genetically different people on Earth, they would still share 99.8 percent of their DNA. In fact, we differ from Neanderthals in less than half a percent of our genome.
You have to go back almost 5,000 years into the past to find the last major migration movement that altered the DNA of all Europeans. The DNA of people who came from the Eastern European steppes 5,000 years ago is still one of the three dominant genetic components on the continent today. The other two originate from early hunter-gatherers and from farmers who migrated there from Anatolia. The genetic ratio of these three archaic populations can be quantified through DNA testing in every person who has European roots.
We developed sweat glands, a more effective cooling system that allowed less hairy archaic hominins to run farther, hunt better, and escape from predators more effectively, meaning they lived longer and had better odds of reproducing. Archaic humans with genes predisposing them to hairiness, on the other hand, less able to compete for resources and outrun prey, died out.
In the time it takes you to read this chapter, the DNA in millions of your cells will undergo chemical changes—in your skin, in your gut, everywhere. Usually these changes are immediately corrected by the body, but not always. When this process goes awry, it’s called a mutation. If mutations appear during the formation of germ cells—that is, in sperm or egg cells—they can be passed on to the next generation. The body has mechanisms to prevent this; as a result, fertilized germ cells with mutations that cause serious illnesses usually die. But smaller mutations often slip through the net, and a genetic change can thus, under certain circumstances, become hereditary.
The human genome consists of 3.3 billion base pairs. In 2003, when the Human Genome Project came to an end, it would have taken more than ten years to unravel the genetic code of a particular individual. Today our laboratory can process a trillion base pairs every day. The throughput of these machines has increased by a factor of 100 million over the past fifteen years: currently, one sequencer can decode an extraordinary 300 human genomes in a single day.
Duygusal İyileşme
Prof. Mansuy: "Eğer genlerimiz harddisk ise epigenetik harddiskteki yazılımdır."
Sayfa 31 - Gün YayıncılıkKitabı okudu
