Why animals diversified on Earth: Cancer research provides clues

Summary: Can tumors teach us about animal evolution on Earth? Researchers believe so and now present a novel hypothesis of why animal diversity increased dramatically on Earth about half a billion years ago. A biological innovation may have been key.

Can tumors teach us about animal evolution on Earth? Researchers believe so and now present a novel hypothesis of why animal diversity increased dramatically on Earth about half a billion years ago. A biological innovation may have been key.

A transdisciplinary and international team, from Lund University in Sweden and University of Southern Denmark presents their findings in Nature Ecology and Evolution.

The new hypothesis holds that the dramatic diversification of animals resulted from a revolution within the animals’ own biology, rather than in the surrounding chemistry on Earth’s surface.

Life on Earth was dominated by microbes for roughly 4 billion years when multicellular life suddenly — then in the form of animals in robust ecosystems — made a vigorous entry. Why animals diversified so late and so dramatically has remained unresolved and is a matter of hot debate.

The diversification of animals occurred over a geologically short period of time and is known as the Cambrian explosion. Many geologists have assumed that the Cambrian explosion was triggered by an increase of atmospheric oxygen.

However, a causal relationship between the Cambrian explosion and increasing atmospheric oxygen lacks convincing evidence.

Historic focus on high oxygen

Indeed, research over the last years weaken the support for a correlation between the Cambrian explosion and increasing atmospheric oxygen. For example, dramatic changes in atmospheric oxygen are noted both before and after the Cambrian, but not specifically when animal diversification took off.

Simple animals are furthermore noted to require surprisingly low oxygen levels, which would have been met well before the Cambrian.

“A heated hunt for the geochemical evidence that oxygen increased when animals diversified goes on but, after decades of discussion, it seems worthwhile to consider the development of multicellularity also from other angles,” says geobiologist Emma Hammarlund, PhD and researcher at the division for translational cancer research at Lund University and guest researcher at the Nordic Center for Earth Evolution at the University of Southern Denmark.

Tumors are successful versions of multicellularity, also at low oxygen

In order to understand more about the conditions for multicellular life, Emma Hammarlund contacted tumor biologist, Professor Sven Påhlman at the Department of Laboratory Medicine at Lund University, who has explored the importance of low oxygen concentrations, or so-called hypoxia, in the tumor setting for nearly two decades.

“I wanted to learn what tumor scientists observe on a daily basis, in terms of tissue growth and how it relates to oxygen. Tumours are after all, and unfortunately, successful versions of multicellularity,” explains Emma Hammarlund.

The team, including also tumor biologist Dr. Kristoffer von Stedingk at Lund University’s Paediatrics division, tackled the historic question of why animals developed so late and dramatically with novel clues from the field of tumour biology.

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