To Start With

Today, it is taken for granted that we are surrounded by varied animal communities that eat one another. Our ecosystems are shaped by relationships between the animals that eat each other, such as killer whales eating seals, who eat squid, which eat krill. To derive energy from their food, many animals and others need oxygen. However, life on Earth didn't always function in this way.

For much of Earth's history, an environment devoid of oxygen and abundant in methane would not have been hospitable to animals. The earliest known living forms are microscopic organisms (microbes), which left traces of their existence in rocks 3.7 billion years ago. The signals were composed of a particular kind of carbon molecule made by living things.

An atmosphere of oxygen

At least 2.4 billion years ago, cyanobacteria developed, paving the way for an extraordinary change. They developed into the planet's first photo-synthesizers, producing food out of water and solar energy while also releasing oxygen. Other microorganisms that couldn't tolerate oxygen found the environment less friendly as a result of the rapid, dramatic spike in oxygen that was caused by this.

Rocks on the seafloor have changed, providing proof of this Great Oxidation Event. Iron oxidizes (reacts chemically with oxygen) and is expelled from the system when oxygen is present. There are iron bands striped on rocks that predate the event. Iron bands are not present in rocks that date from after the event, indicating that oxygen was now present.

Multiple Cell Life

Other breakthroughs, though, were happening. The specialized cells required for complex bodies are not present in bacteria, despite the fact that they can process a wide variety of substances. Skin, blood, and bone are just a few examples of the different cells that make up an animal's body. These cells contain organelles, and each one of these organelles has a specific function. Microbes are merely single cells without nuclei to store their DNA and organelles.

Microorganisms started to live inside other microbes and act as their organelles, which led to a revolutionary change. These mutually advantageous interactions led to the evolution of mitochondria, the organelles that convert food into energy.

First Animals

Eventually, these groups of specialized, interdependent cells gave rise to the first animals, which DNA evidence indicates appeared some 800 million years ago. Among the first animals were sponges. Molecular research suggests that sponges evolved even earlier than the 700 million years indicated by chemical compounds from sponges that have been preserved in rocks.

Even though there was less oxygen in the ocean than there is today, sponges can survive in low-oxygen environments. Although they need oxygen to digest, much like other creatures, they don't need a lot of it because they aren't extremely active. They consume food while remaining motionless by filtering food particles out of water that is circulated throughout their bodies by specialized cells.