First multicellularity: new findings and theories (Evolution)

by David Turell , Wednesday, July 17, 2019, 19:04 (1744 days ago) @ David Turell

A review of current thought, starting with the recognition that single-celled forms were highly complex before multicellularity appeared:

https://www.quantamagazine.org/scientists-debate-the-origin-of-cell-types-in-the-first-...

"The recent work paints a picture of ancestral single-celled organisms that were already amazingly complex. They possessed the plasticity and versatility to slip back and forth between several states — to differentiate as today’s stem cells do and then dedifferentiate back to a less specialized form. The research implies that mechanisms of cellular differentiation predated the gradual rise of multicellular animals. (my bold)

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"In the 2000s, more than a century after Haeckel proposed his theory, genomic evidence confirmed that choanoflagellates were animals’ closest living relatives. “Out of the many single-cell eukaryotes out there, 150 years ago choanoflagellates had been proposed as a close relative of animals,” said Pawel Burkhardt, a molecular biologist at the Sars International Center for Marine Molecular Biology in Norway. “Then the first genome was sequenced, and bam! It actually was really true.”

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"But uncertainty about that clear and elegant story has been growing over the past decade. The idea that animals arose from a colony of choanoflagellate-like cells implies that cell differentiation evolved after multicellularity did. But “the data is demonstrating that it’s not like that,” said Iñaki Ruiz-Trillo, an evolutionary biologist at the Institute of Evolutionary Biology in Barcelona.

"The first complication came in 2008, when a group of scientists, in an effort to more precisely map out the evolutionary relationships among animals on the tree of life, identified comb jellies rather than sponges as the earliest animals. The finding generated controversy. “It’s still very much a heated question,” Gold said, “but I think it forced the community to reappraise the classic narrative.”

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"Back in 1949, the Russian biologist Alexey Zakhvatkin had proposed that multicellular animals evolved when temporally differentiating cells formed colonies and began to commit to particular stages in their life cycles, allowing a few cell types to exist at once. Ruiz-Trillo and his colleagues provided further evidence for this so-called temporal-to-spatial transition. In a series of studies, they showed that certain families of regulatory proteins supposedly unique to animals, including those involved in cell differentiation, were actually already present in their far more ancient unicellular relatives.

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"They expected to establish that sponge choanocytes had gene expression profiles most like those of choanoflagellates. Instead, they found that another type of sponge cell did.

That cell type, called an archaeocyte, acts like a stem cell for the sponge: It can differentiate into any other cell type the animal might need. Some of the gene expression patterns in archaeocytes are significantly similar to those of the protists during particular life cycle stages, according to Bernard Degnan. “They’re expressing genes that suggest that they have an ancestral regulatory system,” he said. “All animals are just variations on that theme that was created a long time ago.”

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"According to some experts, we can think of the single-celled organisms that came before animals as stem cells of sorts: They could go on dividing forever, and they could perform a variety of functions, including reproduction. Other early animals, such as jellyfish, show a great deal of that seemingly ancestral plasticity as well.

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" In a preprint they posted on biorxiv.org in May, Burkhardt and his colleagues found that the cells in a choanoflagellate colony are not all identical: They differ in their morphology and in the ratio of their organelles. These observations, he said, suggest that spatial cell differentiation was already happening in the choanoflagellate lineage, and perhaps even earlier — a possibility that blends the new ideas (that the capacity for differentiation is ancient and the transition to animal multicellularity was gradual) with the old (that this could happen with choanoflagellate-like cells)."

Comment: Note my bold in the first quote which tells us that single-celled early forms were highly complex. This implies to me that the very first life cells were highly complex, and therefore had to be designed. The basis for eventual multicellularity was designed into those first cells