Did the early increase in oxygen on Earth support the evolution of multicellular life – or did it suppress it?


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Scientists have long believed that there was a direct link between the increase in atmospheric oxygen, which began with the great oxygenation event 2.5 billion years ago, and the rise of large, complex multicellular organisms.

This theory, “the oxygen control hypothesis,” suggests that the size of these early multicellular organisms was limited by the depth at which oxygen could diffuse into their bodies. The hypothesis makes a simple prediction that has been very influential in both evolutionary biology and geosciences: greater atmospheric oxygen should always increase the size to which multicellular organisms can grow.

This is a hypothesis that has proven difficult to test in the laboratory. Yet a team of Georgia Tech researchers have found a way – using directed evolution, synthetic biology, and mathematical modeling – all implemented on a single multicellular life form called “snowflake yeast.” The results? Important new information on the correlations between oxygenation of early Earth and the rise of large multicellular organisms – and it all depends on exactly how much O2 is available to some of our early multicellular ancestors.

“The positive effect of oxygen on the evolution of multicellularity is entirely dose-dependent – the first oxygenation of our planet would have strongly constrained, not favored, the evolution of multicellular life”, explains G. Ozan Bozdag, researcher at the Faculty of Biological Sciences. and the lead author of the study. “The positive effect of oxygen on multicellular height can only be realized when it reaches high levels.”

“Oxygen suppression of macroscopic multicellularity” is published in the May 14, 2021 edition of the journal Nature communications. Bozdag’s co-authors on the paper include Georgia Tech researchers Will Ratcliff, associate professor in the School of Biological Sciences; Chris Reinhard, associate professor in the School of Earth and Atmospheric Sciences; Rozenn Pineau, Ph.D. student at the School of Biological Sciences and the Interdisciplinary Graduate Program in Quantitative Biosciences (QBioS); with Eric Libby, assistant professor at Umea University in Sweden and the Santa Fe Institute in New Mexico.

Evolve the yeast in record time

“We show that the effect of oxygen is more complex than previously imagined. The early rise in global oxygen should in fact strongly constrain the evolution of macroscopic multicellularity, rather than selecting organisms. bigger and more complex, ”notes Ratcliff.

“People have long believed that oxygenation of the Earth’s surface is useful – some going so far as to say it is a prerequisite – for the evolution of large, complex multicellular organisms,” he adds. “But no one has ever tested this directly, because we haven’t had a model system that can both quickly undergo many generations of evolution and grow over the full range of oxygen conditions,” the conditions said. anaerobic under modern conditions. levels.

The researchers were able to do this, however, with snowflake yeast, simple multicellular organisms capable of rapid evolutionary changes. By varying their growing environment, they evolved snowflake yeast for over 800 generations in the lab with selection for a larger size.

The results surprised Bozdag. “I was amazed to see that multicellular yeasts doubled in size very quickly when they couldn’t use oxygen, whereas populations that grew up in a moderately oxygenated environment showed no increase in size,” he says. . “This effect is robust, even over much longer time scales.”

Size and oxygen levels are important for multicellular growth

In the team’s research, “tall stature easily grew either when our yeast lacked oxygen or in abundance, but not when oxygen was present at low levels,” says Ratcliff. “We’ve done a lot more work to show that this is actually a completely predictable and understandable outcome of the fact that oxygen, when limiting, acts as a resource – if cells can access it. , they get a great metabolic benefit. When oxygen is scarce. , it cannot diffuse very far in organisms, so there is an evolutionary incentive for multicellular organisms to be small – allowing most of their cells to access oxygen – a constraint that is not there when oxygen is simply not present, or when there is enough around to diffuse deeper into the tissues. “

Ratcliff says his group’s work not only challenges the oxygen control hypothesis, it also helps science understand why so little apparent evolutionary innovation was happening in the world of multicellular organisms in the billion. years after the great oxygenation event. Ratcliff explains that geologists call this period the “boring billion” in Earth’s history – also known as the dullest time in Earth’s history and Earth’s Middle Ages – a time when l Oxygen was present in the atmosphere, but at low levels, and multicellular organisms remained relatively small and uncomplicated.

Bozdag adds another insight into the unique nature of the study. “Previous work has examined the interaction between oxygen and multicellular size primarily through the physical principles of gas diffusion,” he says. “While this reasoning is essential, we also need an inclusive consideration of the principles of Darwinian evolution when studying the origin of complex multicellular life on our planet.” Finally, being able to advance organisms through many generations of evolution has helped researchers accomplish just that, adds Bozdag.

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More information:
G. Ozan Bozdag et al. Removal of oxygen from macroscopic multicellularity, Nature communications (2021). DOI: 10.1038 / s41467-021-23104-0

Provided by Georgia Institute of Technology

Quote: Did the early surge of oxygen on Earth support the evolution of multicellular life – or did it suppress it? (2021, May 17) retrieved May 17, 2021 from https://phys.org/news/2021-05-earth-early-oxygen-evolution-multicellular.html

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