When was the first time life began to predate on each other?

Using the word predation may seem surprising when we talk about the first organisms that set out to eat other organisms, for they were not deadly predators with sharp teeth and claws, but small single-celled life forms that swam around in the primordial sea.

They had neither a mouth nor a gut system; all they had was a cell membrane so soft that they could engulf another, smaller organism, that they encountered on their way.

Thus, the "prey" was not eaten in the way we usually think about; instead it was rather encapsulated – but nevertheless, it provided its “predator" with nutrients and energy.

After this happened there was no going back to a world without predators. But when did it happen?

First primitive life

The question occupies Professor Donald E. Canfield, who has spent much of his career studying how life evolved on Earth.

- That question is inextricably linked to the question of when and how the evolution of life changed the ecosystems of the oceans, he said.

Canfield adds that an answer will not only satisfy our curiosity about our primordial origin:

- It is also about understanding how chemistry and biology interact to control modern marine ecosystems so that we can better predict how the oceans will react to man-made activities and global climate change.

Let's start in the peaceful primordial sea, where organisms had not yet begun to feed on each other.

The dominant life forms were the primitive prokaryotes. They get this name because they have no cell nucleus (pro means before, and karyot is a derivation of the Greek word for nucleus; karyon).

New player enters the field

Then, a new player entered the field. The eukaryotes. Unlike the prokaryotes, eukaryotes have a nucleus and organelles with specialized functions. Their name is created from the two Greek words for genuine (eu) and karyon (core).

The eukaryotes changed everything:

- They could feed on other organisms. And when you introduce eating of other organisms into an ecosystem, it radically changes the dynamics of the system, said Don Canfield.

The eukaryotes today include the life forms that we call advanced: plants and animals (including humans), but back then, in the primeval sea, they were still single-celled organisms.

- But they had this trick; they could engulf organisms that were smaller than themselves. The prey attaches to the cell of the eukaryote after which a small sac forms around the prey cell. In this way the single-celled eukaryote predator absorbs its prey, Canfield says and elaborates:

- They could gain more energy by this process, and what happens when you get more energy? You can grow in size, and that's what happened to the eukaryotes, explained Don Canfield.

So, when did eukaryotes start to feed on other organisms and what were the consequences for marine ecosystems? These are some big questions in understanding the history of life on Earth.

Large 1,7 billion years old fossils

Some clues come from fossils, tangible proof that some form of life actually existed, but what form of life do ancient eukaryote fossils represent?

- In truth, it is very difficult to unravel what type of lifestyle an ancient eukaryote fossil represents, but one thing we can be sure about is their size, and beginning about 1700 million years ago, ancient eukaryotes fossils are large, said Canfield and elaborates:

- Well, maybe not large compared to what you might consider large, but at 100 to 400 microns in size (a human hair is about 70 microns thick), they are big by the standards of single-celled organisms. Consider that most prokaryotes in the ocean are less than 1 micron in size, said Don Canfield.

Still, the size of these fossils can tell us something about the nature of the ancient ecosystems where these organisms lived.

Not a friendly ocean

Using an ecosystem model (developed by Ken Andersen from the Danish Technological University), SDU marine biologist Lisa Eckford-Soper, together with Don Canfield, Trine Frisbæk Hansen and Ken Andersen, found that an ecosystem containing large organisms as found in the fossil record must also have contained eukaryote predators.

This paper can be found here

Even more, modelling suggests that eukaryotes were likely abundant in the ancient ecosystems where the large eukaryotes are found.

So, a combination of new biomarker work (see sidebar) and modelling shows that active eukaryote ecosystems containing algae, but also predators, populated the oceans as far back as 1700 million years ago.

- This is a billion years earlier than previously thought said Lisa K. Eckford-Soper, adding:

- A billion years where marine organisms could feed on each other and therefore, the oceans were not as friendly as we previously imagined.