Friday, July 26, 2013

Animals' Early Missing Link: Mesomycetozoeans

"In China's Guizhou Province, a thin band of rock crowded with fossils smaller than poppy seeds may reveal the origin of all the animals that ever lived."

Prenote: "Without integrin genes (integrins), you don't get cellular cooperation. Cooperation is a fundamental aspect of evolution. Without it, you don't get construction or complexity in life."

"For decades the Chinese mined a phosphorus-rich layer of rock that winds through the hillsides of Guizhou Province and used it to make fertilizer for rice paddies. Then, in the 1980s, several geologists noticed fossils embedded in that layer, which they called the Doushantuo Formation. Viewed under a microscope, the fossils were exquisite—spherical and segmented on their surface, like a blackberry....
Then a paleontology student named Shuhai Xiao brought several hundred pounds of the rock to Harvard there in the 1990s. He noticed that the fossils, though all the same size, consisted of different numbers of cells: one, two, four, eight, and so on, up to around 1,000, roughly doubling with each increase. Xiao knew that it was unusual for a mature organism to stay the same size while undergoing successive rounds of cell division. Animal embryos, however, do so regularly...."

" All major groups of animals—an entire kingdom of multicellular life that today includes insects, worms, shellfish, starfish, sea anemones, coral, jellyfish, and vertebrates like us—bloomed suddenly in the fossil record during an evolutionary extravaganza known as the Cambrian explosion, which occurred 530 million years ago. But genetic studies of modern animals had suggested that all of these creatures evolved from a single-celled ancestor that lived at least 100 million years before that, leaving a huge gap between the estimated origin of animals and the appearance of the earliest known animal fossils. Xiao’s embryos helped fill that gap."

"The fossils also raised new questions. Why had adult specimens of the creatures never been discovered???"

"In 2011, a Doushantuo fossil was subjected to X-ray tomography ?(a 3-D scan) and obtained a beautiful image of its interior. This particular fossil was slightly different from most of the others, peanut-shaped instead of spherical. On its inside the fossil wasn’t crushed at all. It was magnificently preserved, just like Xiao’s embryos—but instead of fewer than 1,000 cells, it contained half a million.... The Doushantuo fossils weren’t embryos after all."

"In an embryo, Donoghue explains, you’d expect tissues, even organs, to develop by the time you have that many cells.” But the numerous cells in the peanut-shaped fossil were not differentiated into organs; they all looked alike. Donoghue now believes the fossils represent a more primitive ancestor of animals. To him they resemble spore sacs formed by modern single-celled organisms called " mesomycetozoeans", which sit between animals and fungi on the tree of life. Mesomycetozoeans reproduce by multiplying inside a sac that eventually ruptures, releasing thousands of individuals. If the Doushantuo fossils are more like spore sacs than like animal embryos, that would explain why no adult animal fossils have turned up."

"Did a Sac of Spores Launch the Animal Kingdom?:
The genomes of several mesomycetozoeans have been sequenced, with surprising results. These single-celled organisms possess genes once thought to be a unique evolutionary invention of animals. Some of the genes control receptors called "integrins", which help cells attach to and communicate with one another, allowing formation of specialized organs and complex body plans .... in the simpler organisms, integrin genes may contribute to the formation of spore sacs."

"What is emerging from this work is a fascinating possible scenario for the humble beginnings of the mighty animal kingdom: A few thousand spore cells in the ocean, poised to scatter like dandelion seeds, may have decided to stick together and continue their experiment in group living for a little while longer. If so, insects, starfish, and even humans may owe their existence to a dust-size spore sac that wouldn’t explode."

Tree of Life