Fossils show early love of sun and sex

U. SHEFFIELD (UK) — Fossils found on remote lochs on the west coast of Scotland are illuminating the key moment in evolutionary time when life made the leap to land.

Some of the ancient fossils that lived on the bottom of the ancient lake bed as long as a billion year ago, are so finely ornamented, and so large and complex, that they are evidence for a surprisingly early start for the emergence of eukaryote cells on land.

It is from these cells that green algae and green land plants—everything from lettuce to larch trees—evolved.

The findings are reported in the journal Nature.

Around 500 million years after the emergence of these complex cells, the surface of the land was starting to become covered in simple vegetation like lichens, mosses, and liverworts, and the first animals were able to take their chance and leave the sea behind.

These pioneers were followed by the first fish and ferns, reptiles, conifers, mammals, flowering plants—and eventually, humans.

“It is generally considered that life originated in the ocean and that the important developments in the early evolution of life took place in the marine environment: the origin of prokaryotes, eukaryotes, sex and multicellularity,” says Charles Wellman, reader in paleobiology at the University of Sheffield.

“During this time the continents are often considered to have been essentially barren of life—or at the most with an insignificant microbial biota dominated by cyanobacteria.

“We have discovered evidence for complex life on land from 1 billion year old deposits from Scotland.

“This suggests that life on land at this time was more abundant and complex than anticipated. It also opens the intriguing possibility that some of the major events in the early history of life may have taken place on land and not entirely within the marine realm.”

“These new cells differ from their bacterial ancestors in that they have specialized structures including a nucleus, as well as mitochondria and chloroplasts, which are vital for photosynthesis,” says Martin Brasier, professor of earth sciences at the University of Oxford.

“They also undergo sexual reproduction, leading to much more rapid rates of evolutionary turnover.

“It may even be that the sort of conditions found in the ancient lakes around Loch Torridon favored a key step in this transformation, which involved the incorporation of symbiotic bacteria into the cell to form chloroplasts, rather than this occurring in the sea as usually envisaged.”

“None of this would have been possible without advances long ago made by these little microbes, now entombed within phosphate from the Torridon lakes. It was arguably these organisms that helped to turn our landscape from a harsh and rocky desert into a green and pleasant place.”

Discovered cell pairs include a nucleus, mitochondria and chloroplasts. (Credit: Martin Brasier, U. Oxford)

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