Skapa lösning till fråga 37 ELF

Våren 2017


Our True Dawn

Line them up in your head. Generation after generation of your ancestors, reaching back in time through civilisations, ice ages and epic migration out of Africa, to the very origin of our species. And on the other side, take a chimp and line up its ancestors. **How far back do you have to go, how many generations have to pass, before the two lines meet?** We know that at some point we shared a common ancestor, but exactly when – and what that ancestor was like – have been maddeningly hard to pin down. Palaeontologists have searched for fossil remains, and geneticists have rummaged through the historical documents that are human and chimp DNA. Both made discoveries but they did not see eye to eye. The obvious first place to look for answers is in the fossil record. But fossil humans are notoriously thin on the ground and difficult to interpret. Geneticists have more to work with. DNA contains telltale traces of events in a species’ past, including information about common ancestry and speciation. As two species diverge from a common ancestor, their DNA becomes increasingly different, largely due to the accumulation of random mutations. The amount of genetic difference between two related species is therefore proportional to the length of time since they diverged. To estimate when the human–chimp split occurred, geneticists can simply count the differences in matching stretches of chimp and human DNA and divide it by the rate at which mutations accumulate. This is known as the molecular clock method. But there is a catch. To arrive at the answer, you have to know how fast the mutations arise. And that leads you back to square one: you first need to know how long ago we split from chimpanzees. To get around this catch-22, geneticists turned to orangutans. Fossils suggest that they split from our lineage between 10 and 20 million years ago. Using this fudge, geneticists arrived at a mutation rate of about 75 mutations per genome per generation. In other words, offspring of humans and chimps each have 75 new mutations that they did not inherit from their parents. This, in turn, led to a guess that human ancestors split from chimpanzees between 4 and 6 million years ago. When fossil-hunters hear this number, they cry foul. “Geneticists ignored the palaeontologists completely.” says Owen Lovejoy of Kent State University in Ohio. “To claim a 4 million divergence date is just silly.” Simply put, the palaeontologists were sure, on the basis of fossil evidence, that there was little chance the DNA results were accurate. Humanity, they affirmed, had to be older than the geneticists claimed. History looks set to prove them right. In the past few years, researchers studying human populations have for the first time been able to observe mutations almost as they happen. Instead of relying on an estimate based on rare fossils, we can now watch the molecular clock ticking in real time. In 2012, Augustine Kong of Decode Genetics in Reykjavik, Iceland, and colleagues published a ground-breaking study. After scanning the genome of 78 children and their parents to count the number of new mutations in each child’s genome, they found that each child carries an average of 36 new mutations. Crucially, that is half what was previously assumed, meaning the molecular clock ticks more slowly than we thought, pushing the human–chimp split further back in time. This has consequences for other aspects of human prehistory. In particular, early genetic estimates suggested that our ancestors left Africa 50,000 years ago. So when fossil remains in Israel and archaeological sites in India were found to be around 100,000 years old, there was some explaining to do. The Israeli bones were dismissed as the remains of an early dead-end excursion, and the Indian sites as an error, pure and simple. The new molecular clock resolves the discrepancy, pushing the departure from Africa back to between 90,000 and 130,000 years ago. Other key events await revision. But the main finding is clear. The human lineage is significantly older, and our closest living relatives more distant, than we once thought. We are used to thinking about ourselves as separate and distinct from the rest of the animal kingdom. We just got a bit more separate, and a bit more distinct.

**Which of the following statements about the “molecular clock method” is true, ­ according to the text?**

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