600 Million Years Ago, the First Complex Life On Earth Grew to Colonize the Planet

Why did some of Earth's earliest complex organisms start to grow larger?

A study published in the journal Nature Ecology & Evolution may provide an answer to this question, suggesting that organisms grew bigger to spread their offspring as far as possible, rather than to compete for resources as was previously thought.

Researchers found that the most successful organisms in Earth's oceans half a billion years ago were those that were capable of spreading their offspring the furthest.

The very first organisms on Earth were simple and microscopic. But during the Ediacaran Period (635 to 541 million years ago), a wonderful variety of larger, more complex lifeforms first began to emerge.

In the study, the scientists studied a type of organism known as "rangeomorphs" which grew up to two meters tall and resembled ferns. Despite their similarities to plants, these may actually have been some of the first animals ever to exist.

But determining where Ediacaran organisms fit on the tree of life has proven challenging. Some scientists have expressed doubt as to whether many of them can even be classed as animals.

Ediacaran organisms don't appear to have mouths or organs, and they are not thought to have been capable of moving, instead absorbing nutrients from the water around them. Most of the larger examples are quite distinct from later life-forms.

Over time, many of these Ediacaran organisms grew taller and their body shapes diversified, with some growing stem-like structures to support themselves.

In many environments, such as forests, there is intense competition for resources such as sunlight, so taller trees and plants tend to have an advantage. The research team, led by Emily Mitchell from the University of Cambridge, wondered whether a similar mechanism was driving growth in Ediacaran lifeforms.

"We wanted to know whether there were similar drivers for organisms during the Ediacaran period," Mitchell from Cambridge's Department of Earth Sciences, said in a statement. "Did life on Earth get big as a result of competition?"

For the study, the team examined fossils from Mistaken Point in Newfoundland, Canada—a globally significant fossil site which contains numerous specimens from the Ediacaran Period.

Previous studies have hypothesized that an increased organism size was driven by competition for nutrients at different water depths in the Ediacaran oceans. However, the new study shows that a lack of nutrients may not have been an issue.

"The oceans at the time were very rich in nutrients, so there wasn't much competition for resources, and predators did not yet exist," Mitchell, said. "So, there must have been another reason why life forms got so big during this period."

Because Ediacaran organism could not move, they are generally preserved where they lived, something which is beneficial to researchers because it enables them to analyze whole populations from the fossil record.

The team's analysis showed that there was no correlation between the height of organisms and competition for food. They found that organisms did not live at different water levels to avoid competing for resources—a phenomenon known as tiering.

"If they were competing for food, then we would expect to find that the organisms with stems were highly tiered," said Charlotte Kenchington, co-author of the study from Memorial University of Newfoundland. "But we found the opposite: The organisms without stems were actually more tiered than those with stems, so the stems probably served another function."

The researchers suggest that one likely function of these stems would have been to enable the greater dispersion of offspring. Rangeomorphs, for example, spread their offspring by expelling small structures known as propagules. They found that the tallest organisms were surrounded by the most offspring, suggesting that this trait led to a greater chance of colonizing an area.

Ediacaran fossils at Mistaken Point, Newfoundland. Emily Mitchell

"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."

Ediacaran organisms represent an important milestone in the evolution of life on Earth because they immediately precede the so-called Cambrian Explosion—a rapid increase in biodiversity when most of the major animal groups appear in the fossil record.

It is unclear how Ediacaran lifeforms relate to modern animals, because they largely disappeared with the Cambrian explosion. Macroorganisms—organisms that can be seen with the unaided eye—from the Ediacaran appear to have been completely replaced by those that evolved during the explosion, for example. And most of the body plans of existing animals derive from creatures that first appeared in the Cambrian fossil record.