Evolution and humans; our unique genes (Evolution)

by David Turell @, Wednesday, October 21, 2015, 14:01 (1398 days ago) @ David Turell

Appearing in the last 3+ million years:

http://www.theatlantic.com/science/archive/2015/10/the-mystery-of-human-only-genes-and-...

"One such unique human gene is HYDIN2. It first appeared around 3.1 million years ago, as a duplicate of an existing gene called HYDIN. During the duplication process, “the head got chopped off and the tail got chopped off,” explains Max Dougherty from the University of Washington. It was as if someone had transcribed a book but neglected the prologue and epilogue. That should have been a fatal mistake since the prologues of genes contain sequences called promoters, which switch them on or off. The new gene should have been dead on arrival—a book that couldn't be opened.

"Instead, as luck would have it, it fused with a copy of another gene, which gave it a new lease on life. The fusion, which Dougherty described at the American Society of Human Genetics 2015 conference, created an entirely original gene, which looks like HYDIN but with a new prologue and a new first chapter. And while HYDIN, like most of our genes, exists in many other animals, its wayward daughter—HYDIN2—is a human-only innovation. (my bold)

***

"Duplicated genes make up some 5 percent of the human genome. Many of them have arisen in the last 10 to 15 million years, since humans, chimps and gorillas started going our separate evolutionary ways. In fact, we—the great African apes—have ended up with far more duplicated genes than, say, orangutans or macaque monkeys. No one fully understands why.

"What's clearer is that these genes are organized in a very unusual way. For example, in other mammals like elephants, rats, and platypuses, the copies tend to sit next to the originals in a tandem series. But in humans, chimps, and gorillas, they disperse across the genome.

"They also have a unique architecture. Imagine a gene, G1, which gets copied into a different part of the genome, producing G2. Now, another duplication event copies G2, creating yet another copy of G1 along with some of the new DNA surrounding it. This happens again and again; with each new duplication event, the core genes picks up more flanking material. “It builds an inverse Oreo cookie,” he says, while holding his hands out and pulling them further and further apart.

***

"So, it takes a lot of work to even discover these genes, let alone divine their function. For example, in 2010, Eichler's team identified 23 human-specific duplicated genes that aren't found in other apes. One of these, SRGAP2, has been duplicated three times, producing copies that aren't found in the reference human genome.

"The second of these, SRGAP2C, is especially interesting. It emerged around 2.4 million years ago, at the time in our evolution when the human brain was becoming distinctively bigger. And Franck Polleux from the Scripps Research Institute showed that SRGAP2C controls the growth and movement of neurons, leading to a thicker set of connections between these cells.

"Marta Florio and Wieland Huttner from the Max Planck Institute of Molecular Cell Biology and Genetics found a similar example earlier this year. They found that a human-specific duplicated gene called ARHGAP11b was exceptionally active in radial glia, a group of stem cells that generate many of the neurons in our developing brains. When the team activated the human gene in embryonic mice, the rodents developed a larger pool of radial glia, and the kinds of deep folds that are typical of a human brain."

Comment: Lucky or designed?


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