Who were the very first animals to set foot on the earth?

Stan Megraw
23 January 2012

Above: Image © Ghedoghedo, Wikimedia Commons

As a kid, many of us were fascinated by dinosaurs (and maybe still are). They were certainly awesome looking creatures, unlike anything we see today. Roaming the earth some 230 million years ago, they are commonly thought to be one of the first animals in the world. But the first animals on earth actually appeared about 400 million years earlier than dinosaurs and they were discovered right here in Canada! Once I reveal their identity to you, you would never have guessed they were animals at all.

But first, a little background.

Learning about Early Life

Before the internet became popular, much of our knowledge came from books. This is especially true when studying history. Because so many people wrote about important events, we have a good record of things like the Confederation of Canada, War of 1812, etc.

Learning about the earliest animals is a very different matter. This is because the world's oldest book is only 2400 years old — a very long time after the first animals showed up.

So how did scientists figure out when animals first existed? By using a "book" published by nature — fossils.

Did You Know? The study of fossils is a branch of science known as palaeontology.

Fossils are the remains of living organisms that have usually been preserved in sedimentary rock. After an animal (or plant) dies, their remains sink to the bottom of a lake or sea. Over time, they are gradually buried under many layers of sediment and eventually become fossilized.

Did You Know? Examples of sedimentary rocks are shale, sandstone and limestone.

Fossilization can occur in many ways. The most common is where minerals (chemicals other than oxygen, carbon, nitrogen or hydrogen) in the body of an animal are dissolved and replaced by other minerals (e.g. silica or calcite). This leaves behind a solid rock-like impression of the original organism.

Aging Fossils

When a fossil is discovered, scientists need to figure out how old it is. There are two ways this can be done:

Absolute Dating

This method uses the decay rate of radioactive isotopes — elements with an unstable atomic nucleus. Scientists know that these elements break down at a certain rate called a half-life — the time needed for half the atoms of a one radioactive isotope to decay and become an isotope of another element.

One example is carbon-14, where the "14" means it contains 6 protons and 8 neutrons. This "parent" isotope gradually changes to the "daughter" isotope nitrogen-14, with a half-life of 5,570 years. By measuring the parent-to-daughter ratio, the age of a fossil can be calculated.

Relative Dating

Relative dating looks at comparative rather than actual ages. One way is by superposition, which is based on the assumption that fossils in lower rock layers are older than those in the higher ones.

Another method uses correlation where rock layers from different regions are compared on the basis of mineral composition, fossil content, etc. If the various features match, the areas are of similar age. Particularly useful are fossils with a limited but defined age range (e.g. dinosaurs).

Did You Know? Using fossils to correlate rock layers is known as biostratigraphy.

What Actually is an Animal

This is the final topic we need to briefly cover. An animal is anything that a) has their DNA in a nucleus; b) requires carbon-containing organic molecules for growth; and c) is multicellular. In other words, it doesn't necessarily have to be something with eyes, ears, legs etc.

And now we come to the answer.

The first multi-celled animals evolved about 600 million years ago. Known as primitive metazoa, there are three basic groups: sponges, cnidarians (e.g. corals, sea anemones, and jellyfish) and worms.

In 1990, researchers from Queen's University and University of Montreal discovered the oldest metazoan fossils in the Mackenzie Mountains, Northwest Territories. They are seen here as soft, cup-shaped organisms, similar to sea anemones.

In closing...

So there we have it - the first animal on earth was a marine organism. It was not until 240 million years later that the first animal showed up on land. This was a four-legged amphibian called Ichthyostega, which is a Greek term meaning "fish roof".

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Stan Megraw

Stan is a writer/researcher, a PhD graduate of McGill University and was a member of the CurioCity team for several years. As a kid he dreamed of playing hockey in the NHL then becoming an astronaut with NASA. Instead, he ended up as an environmental research scientist. In his spare time Stan enjoys working on DIY projects, cooking and exploring his Irish roots.


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Comments

Avatar  Chris Gass

This article should be updated to account for the euthycarcinoids that sometimes walked on land during the Cambrian Period in the USA and Canada.

Avatar  vikram kumar

as you so show shall you reap

Avatar  Jack Martin

This is a really interesting article. I never knew animals went that far back in time.

Avatar  Yepo

This is very excite to know the thing

Avatar  paige

What animal is that because thats a cool animal to be the first animal on earth