From the article: [...] [...] [...] [...] [...] Not a lot of evidence in the article. The paper apparently goes into it more, but I didn't find it all that readable.
From the article:
In 1994, scientists didn’t know enough about those microscopic details to develop a hypothesis for how they evolved as well. Three decades later, that’s no longer the case. “There’s lots of molecular data now that we can use that is extremely powerful,” Dr. Nilsson said.
He and other vision experts have now joined forces to develop a hypothesis for how vertebrate eyes evolved.
“You look at all the evidence in your head, and it suddenly clicks,” said Tom Baden, a neurobiologist at the University of Sussex who collaborated with Dr. Nilsson. They and their colleagues unveiled their detailed scenario for the evolution of vertebrate eyes on Monday in the journal Current Biology.
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The scenario starts about 560 million years ago, when our invertebrate ancestors lived mostly buried in the ocean floor. They stuck out their brainless heads to filter bits of food floating by.
On the top of their heads, Dr. Nilsson and his colleagues propose, these forerunners of vertebrates possessed a single patch of light-sensitive cells. Those cells tracked the cycle of day and night, setting the animals’ body clocks, and also provided simple clues about their position, so that the animals could keep their heads just high enough to eat without being eaten.
Some of the descendants of this cyclopean ancestor left their burrows and started to swim. They were still simple creatures with tiny brains, and they still filtered food from the water they swam through. But now they needed more information about their environment.
Their single eye grew more complex. Cup-shaped depressions evolved on either side, sensitive to the direction of incoming light. (Different light-sensitive cells became active depending on where they sat along the curve of the cups.) Dr. Nilsson and his colleagues argue that these were the forerunners of the retinas in our own eyes.
An awareness of the direction of light helped the animals travel through the water, enabling them to stay upright and stable.
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Over millions of years, our filter-feeding ancestors evolved into tiny fish, complete with brains and mouths they could use to catch live animals. Dr. Nilsson and his colleagues contend that this transformation could not have happened without an additional change to the eyes.
“There is a better place for them, on either side of the head,” Dr. Nilsson said.
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But as the new eyes migrated to their new positions, the animals still retained the ancestral eye on top of their heads. While it could not provide details about their surroundings, it continued to provide vital information, such as the overall level of light. Modern fish still have a light-sensitive patch of cells on top of their heads, known as the pineal gland.
“It’s a compelling new idea, but the jury is still out,” said Karthik Shekhar, a computational biologist at the University of California, Berkeley, who was not involved in the study. One way to test the idea, he said, would be to compare the activity of cells in the pineal gland and the retina in many vertebrate species. If Dr. Nilsson and his colleagues are right, the cells in the two organs should have deep molecular similarities — a sign of the deep evolutionary link.
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But new fossil discoveries suggest that the evolutionary course of eyes may have taken some surprising turns that Dr. Baden and his colleagues hadn’t envisioned in their hypothesis.
In recent years, paleontologists in China and England have been studying some of the earliest vertebrate fossils, which date back 518 million years. These show traces of eyes on the sides of the head, complete with lenses and retinas. But at the top of the head, there is a second pair of eyes, complete with lenses and retinas.
Jakob Vinther, a paleontologist at the University of Bristol, speculated that early vertebrates — small animals hunted by big invertebrates — may have benefited from the extra-wide field of view that four eyes provided.
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Dr. Nilsson speculated that the extra eyes might have evolved thanks to a drastic change to vertebrate DNA. Some studies hint that, early in the evolution of vertebrates, the entire genome was duplicated. An extra set of genes may have given rise to an extra pair of eyes.
Not a lot of evidence in the article. The paper apparently goes into it more, but I didn't find it all that readable.
From the article:
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Not a lot of evidence in the article. The paper apparently goes into it more, but I didn't find it all that readable.