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Why life got big

New research shows early organisms grew big to spread offspring

Research

By Sarah Collins and Kelly Foss

Some of the earliest complex organisms on Earth — and possibly some of the earliest animals to exist — got big not to compete for food, but to spread their offspring as far as possible.

The research, which appears in the June 25 issue of the journal Nature Ecology and Evolution, was co-authored by Dr. Charlotte Kenchingon, a post-doctoral fellow in Memorial University’s Department of Earth Sciences and Dr. Emily Mitchell of Cambridge University.

The pair examined fossils from Mistaken Point in Southeastern Newfoundland, one of the richest sites of Ediacaran fossils in the world.

They 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 colonizing their surroundings.

First complex organisms

Prior to the Ediacaran period, between 635 and 541 million years ago, life forms were microscopic in size. 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.

Artist reconstruction
The positions, sizes and taxonomic identity of the organisms in this artist’s reconstruction of Lower Mistaken Point are based on real data from the surface. This community was dominated by the rangeomorphs, Beothukis plumosa and Trepassia wardae. As these communities were below the photic zone, lighting depicted is from an artificial source.
Photo: C.G. Kenchington

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.

All-you-can-eat

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.

“We wanted to know whether there were similar drivers for organisms during the Ediacaran period,” said Dr. Mitchell. “Did life on Earth get big as a result of competition?”

Mistaken Point fossils
A group of Ediacaran specimens of Fractofusus and Plumeropriscum from the ‘E’ surface, Mistaken Point Ecological Reserve. Also visible are patches of the volcanic ash that smothered the organisms on the Ediacaran seafloor, preserving all members of the community in their original positions. This in situ preservation means that modern spatial analyses can be successfully used to learn more about their biology in life.
Photo: E. G. Mitchell

Earlier research hypothesized that increased size was driven by the competition for nutrients at different water depths. However, the current work shows that the Ediacaran oceans were more like an all-you-can-eat buffet.

“If they were competing for nutrients, we’d expect the organisms to be tiered,” said Dr. Kenchington. “What we see instead is that being taller allowed their offspring to travel further distances from the parent. So, we think that reproduction was the main reason life got big when it did.”

Reproductive reasons

Since Ediacaran organisms were not mobile and were preserved where they lived, it’s possible to analyze whole populations from the fossil record.

Using spatial analysis techniques, Drs. Mitchell and Kenchington found 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 colonizing 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 Dr. Mitchell. “Instead, reproduction appears to have been the main reason that life on Earth got big when it did.”

Despite their success, rangeomorphs and other Ediacaran organisms disappeared at the beginning of the Cambrian period about 540 million years ago, a period of rapid evolutionary development when most major animal groups first appear in the fossil record.


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