Why life on Earth first got big

The research, led by the University of Cambridge, found that the most successful organisms living in the oceans more than half a billion years ago were the ones that were able to ‘throw’ their offspring the farthest, thereby colonising their surroundings.

Life in the Ediacaran sea. Image credit: Maulucioni/Wikimedia Commons (CC BY-SA 4.0)

Prior to the Ediacaran period, between 635 and 541 million years ago, life forms were microscopic in size, but during the Ediacaran, large, complex organisms first appeared, some of which – such as a type of organism known as rangeomorphs – grew as tall as two metres.

These organisms were some of the first complex organisms on Earth, and although they look like ferns, they may have been some of the first animals to exist – although it’s difficult for scientists to be entirely sure.

Ediacaran organisms do not appear to have mouths, organs or means of moving, so they are thought to have absorbed nutrients from the water around them.

As Ediacaran organisms got taller, their body shapes diversified, and some developed stem-like structures to support their height.

In modern environments, such as forests, there is intense competition between organisms for resources such as light, so taller trees and plants have an obvious advantage over their shorter neighbours.

Dr Emily Mitchell of Cambridge’s Department of Earth Sciences and Dr Charlotte Kenchington from Memorial University of Newfoundland in Canada examined fossils from Mistaken Point in south-eastern Newfoundland, one of the richest sites of Ediacaran fossils in the world.

They found that there was no correlation between height and competition for food. Different types of organisms did not occupy different parts of the water column to avoid competing for resources – a process known as tiering.

According to the researchers, one likely function of stems would be to enable the greater dispersion of offspring, which rangeomorphs produced by expelling small propagules. The tallest organisms were surrounded by the largest clusters of offspring, suggesting that the benefit of height was not more food, but a greater chance of colonising an area.

“While taller organisms would have been in faster-flowing water, the lack of tiering within these communities shows that their height didn’t give them any distinct advantages in terms of nutrient uptake,” said Mitchell. “Instead, reproduction appears to have been the main reason that life on Earth got big when it did.” (University of Cambridge)

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