In order for animals and plants to evolve, conditions on earth had to be just right – there needed to be enough oxygen in the air, volcanoes had to stop spewing dangerous lava, and the Earth had to cool down so that a rocky crust could form.
At first, life existed only in the ocean. The first life on earth was probably simple, one-celled organisms that lived near hot vents on the ocean floor. But once conditions were right, early plants and animals evolved, like algae, corals, sponges, and the first fish.
During this time in Earth’s history – called the Paleozoic Era – animals made a huge change: they moved from the water to the land.
But switching from life underwater to living on land was no easy task. It took many millions of years for animals to go from “fins to feet.”
By about 450 million years ago, plants were adapting to live in shallower and shallower water. All those plants made the edges of lakes thick and murky, making it hard for water-dwelling creatures to breathe. Because of these challenging conditions, fish evolved lungs, which helped them gulp oxygen from the air instead.
All that muck also made it tough to swim in shallow waters! Over many millions of years, some fish evolved limbs to help them crawl along the bottom. Eventually, these animals could support themselves enough to lift their head out of the water, and some had limbs that were strong enough to help them creep out of the water completely. These creatures had hit upon the right combination of traits, making them the very first animals to walk on land.
Before animals made the leap onto land, they faced steep competition in the water. Water-dwelling animals all had to compete with each other for limited food and space. In addition, they also had to watch out for underwater predators.
But the earliest animals that lived on land instead suddenly didn’t have to compete anymore. Since they were the first animals up on dry land, they had plenty of space and all the food they could eat. They also didn’t have to worry about being gobbled up – at least until other predatory species evolved adaptations that allowed them to move onto land too. So in many ways, those first land-dwellers had a big advantage over other species that still lived in the water.
“Living fossils” are animals and plants that haven’t changed a whole lot since their ancient relatives roamed the Earth. This doesn’t mean they aren’t still evolving – they’re just still recognizable from these ancient forms.
The first horseshoe crabs appeared almost 450 million years ago during the Paleozoic Era – way before the earliest dinosaurs! The horseshoe crabs we know today are very similar to their early ancestors, which managed to survive every big extinction in Earth’s past.
Horseshoe crabs aren’t really crabs at all – they’re more closely related to spiders and scorpions. They may look scary, but they generally aren’t dangerous to people. Their hard shells and spikes protect them from predators , and their long tails help them flip over when they’ve been overturned by waves on the beach.