Evolution of the Human Eye

Fossil Evidence of an Early Form of Vertebrate Eye

Ancient armored fish fossils from Australia present some of the first definite fossil evidence of a forerunner to the human eye.

Fossils of an ancient armored fish, the placoderm, around 400-million-years-old, have provided evidence, via computer X-ray tomography, of the possible evolution of the human eye.

The researcher who made the discovery, Dr. Gavin Young from the Australian Nation University’s Department of Earth and Marine Sciences, analyzed fossilized remains of 400-million-year-old Devonian placoderms – jawed ancestors of modern fish whose bodies were protected by thick bony armor.

The fossils were found near Lake Burrinjuck in New South Wales.

“The ancient limestone reefs exposed around Lake Burrinjuck in New South Wales have produced exceptionally well-preserved placoderm specimens with the braincase intact,” Dr. Young said.

Fossil evidence for the evolution of the eye

The palaeobiologist discovered that unlike all living vertebrate animals – from the jawless lamprey fish to humans – placoderms had a different arrangement of muscles and nerves supporting the eyeball, evidence of an “intermediate stage” between the evolution of jawless and jawed vertebrates.

Proponents of creationism or ‘Intelligent Design’, often cite the vertebrate eye as something so complex and structurally perfect that it could not have evolved.

“The fossil record has something to say about the evolution of the eye, as here we have a superb example showing that the complexity of the eye goes back 400 million years and when we look in detail we find an intermediate stage between the jawed and jawless fish,” he said.

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“The whole argument is, can you evolve something as complex as an eye from a series of small steps, and this fills in a major gap in the evolutionary sequence that has led to the eyes of all living species.

Bony cavities retained imprints

“Part of the trouble in tracing the evolution of the eye is that soft tissues don’t tend to fossilize,” Dr Young said.

“But the eye cavities in the braincase of these 400 million-year-old fossil fish were lined with a delicate layer of very thin bone. All the details of the nerve canals and muscle insertions inside the eye socket are preserved – the first definite fossil evidence demonstrating an intermediate stage in the evolution of our most complex sensory organ.”

The placoderm fossils were analyzed using computer X-ray tomography at ANU, a scanning technique that creates a three-dimensional image of complex organic structures.

Eyeball arrangement similar to sharks’

It showed an anatomical arrangement similar to that in modern sharks, and different from all modern vertebrates, in which there is a consistent pattern of tiny muscles for rotating each eyeball.

“These extinct placoderms had the eyeball still connected to the braincase by cartilage, as in modern sharks, and a primitive eye muscle arrangement as in living jawless fish,” Dr. Young said.

Intermediate form of eye 400 million years old

“What this research shows is that 400 million years ago there was already a complex eye and one that was an intermediate form between jawless and jawed vertebrates.

“This means that we’re able to add one more piece to the puzzle of how the human eye came to be.”

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Dr. Young’s findings are published in the journal, Biology Letters, a publication of The Royal Society, London.

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