Why Bigger Is Better: MUN Biologist Challenging Former Theory on, “Why Life on Earth Got Big”

Competition for resources has been the long held theory as to why life forms of our planet got so big. Until now. Could enhanced dispersal for better colonization be the answer?

Old things are nothing new to us in Newfoundland & Labrador: St. John’s is the oldest port city in North America, for instance, and the Regatta is our country’s oldest running sporting event.

Another of our oldest things is in the news again this week: Mistaken Point ecological reserve. Fossils contained in its rock beds date way back in time to an era when all life on earth lived in the sea (580 to 541 million years ago).

Now a World Unesco Heritage site, Mistaken Point was also the stomping grounds for a pack of scientists, as they set out to solve a burning question in their field: why did animals get so big? The earliest animals on earth were nearly invisibly small.

It turns out animals may haven grown in size to increase our ability to spread out and colonize more area. So goes the theory in the article, “The Utility of Height for the Ediacaran Organisms of Mistaken Point,” recently published in Nature Ecology & Evolution, and co-authored by Dr. Charlotte Kenchingon, a post-doctoral fellow in Memorial University’s Department of Earth Sciences, Faculty of Science, and Dr. Emily Mitchell of Cambridge University.

Doctors Kenchingon and Mitchell 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 more effectively. Think of Dandelions – the better you can disperse your seed (or your young) the more of the neighbourhood (or environment) your species can conquer and colonize.

Prior to the Ediacaran period, life forms were microscopic in size. Then, during the Ediacaran period, large, complex organisms started to appear in our fossil record. Some of which — such as a type of organism known as rangeomorphs — grew as tall as two metres.

As fossils, Rangeomorphs look like squat ferns, but it’s believed they were some of the first complex organisms on Earth, and possibly, the first complex animals.

Height = Advantage, But Not the Advantage We’ve Long Assumed

As Ediacaran organisms got taller, their body shapes diversified and some developed stem-like structures to support their height. Dr. Mitchell says the team wanted to know if this growth was borne of competition.

To put this in perspective, picture a forest. There is intense competition between plant life for sunlight in a forest, so taller trees have an obvious advantage over their shorter neighbours.

Earlier research hypothesized that increased size in animals was driven by the same type of competition, but for nutrients at different depths at sea. However, current work shows that the Ediacaran oceans were an all-you-can-eat buffet, so that earlier theory, of animals growing in size to compete for limited nutrients, has been compromised. If rangeomorphs were getting ample nutrients at shallow depths, why shoot up in search of nutrients?

Also, as Dr. Kenchington says, “If they were competing for nutrients, we’d expect the organisms to be tiered.” Tiering, in a nutshell, is a natural occurrence in nature of different types of organisms occupying different parts of the water column to avoid competing for resources.

Since Ediacaran organisms were not mobile, they were preserved as fossils where they lived. This makes it possible to analyze whole populations from the fossil record. Using spatial analysis techniques, Drs. Mitchell and Kenchington found there was no real evidence of tiering going on.

“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.”

According to the researchers, one likely function of sprouting tall stems would’ve been to enable the greater dispersion of offspring. Rangeomorphs reproduced by expelling small propagules. Height would certainly help them propagate and colonize a greater area.

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