First multicellularity: calcified amoeba forms (Evolution)

by David Turell , Thursday, February 28, 2019, 19:35 (2093 days ago) @ David Turell

The story covers research on the development of Thecamoebae, a shelled form of multicellular amoeba like the foraminifera:

https://phys.org/news/2019-02-amoebae-diversified-million-years-earlier.html

"The study, which was supported by São Paulo Research Foundation—FAPESP, revealed eight new ancestral lineages of Thecamoebae, the largest group in Amoebozoa. Thecamoebians are known as testates because of their hard outer carapace or shell.

"Interpretations of the evolution of Earth's atmosphere and climate change are also affected by the discovery that amoebae are more diverse than previously thought.

***

"According to Lahr, the study presents a different view of how microorganisms evolved on the planet. The late Precambrian was considered a period of low biotic diversity, with only a few species of bacteria and some protists.

"'It was in this period 800 million years ago that the oceans became oxygenated. For a long time, oxygenation was assumed to have led to diversification of the eukaryotes, unicellular and multicellular organisms in which the cell's nucleus is isolated by a membrane, culminating in the diversification of macroorganisms millions of years later in the Cambrian," Lahr said.

"The study, he added, focuses on a detail of this question. "We show that diversification apparently already existed in the Precambrian and that it probably occurred at the same time as ocean oxygenation. What's more, geophysicists are discovering that this process was slow and may have lasted 100 million years or so," he said.

"However, scientists do not know what pressure triggered this oxygenation. "Regardless of the cause, oxygenation eventually led to more niches, the eukaryotes diversified, and there was more competition for niches. One way to resolve the competition was for some lineages to become larger and hence multicellular," Lahr said. (my bold)

***

"In addition to the discovery of greater diversity in the Precambrian, the study also innovates by reconstructing the morphology of the ancestors of thecamoebians to establish that the vase-shaped microfossils (VSMs) found in various parts of the world already existed in the Precambrian and even in the major ice ages that occurred during this era.

"VSMs are presumed to be fossils of testate amoebae. They are unicellular and eukaryotic and have an external skeleton. Significant diversity of VSMs has been documented for the Neoproterozoic Era, which spanned between 1 billion and 541 million years ago, and was the terminal era of the Precambrian.

"In addition to the lack of DNA-containing fossils, the researchers faced another obstacle in reconstructing the phylogenetic tree: thecamoebians cannot be cultured in the laboratory, and genetic sequencing by conventional means is therefore ruled out.

"The solution to this problem was to use the single-cell transcriptome technique to analyze phylogenetics (instead of gene expression, its normal application). "We sequenced whole transcriptomes of arcellinid amoebae using live samples," Lahr explained. "This yielded several thousand genes and some 100,000 amino acid sites, or 100,000 datapoints giving us the phylogenetic tree, which had never been seen before."

"The researchers used transcriptome-based methodology to
capture all messenger RNAs from each individual cell and convert them into a sequenceable complementary DNA library."

Comment: No question, lowly amoeba led the way to multicellularity, but as my bold above shows, we recognize that oxygen appeared in larger quantities, but science doesn't know what drove the changes. But the Cambrian Explosion gap is not any smaller.