Life on Earth began from basic, single-cell organisms. But today, we have a dizzying array of plants and animals on land and in our waters. There are still single-cell organisms, but many are now much more diverse. How did this change in Earth’s species happen over time? Earth’s geology and climate directly impacted the evolution of life on Earth.

What was life like on Earth billions of years ago? With frequent volcanic eruptions, the atmosphere was thick and poisonous. With little oxygen in the air, no life forms as we know them today could survive. Over time, the fiery lava on Earth’s surface began to cool, forming a solid crust surrounded by water. The atmosphere still lacked oxygen, and the heat on the planet was intense. Without oxygen in the atmosphere, there was no ozone layer around Earth, and dangerous ultraviolet rays from the Sun baked the planet’s surface.

The seas at that time were so hot, they routinely boiled! And in those boiling seas, life began to form. The fossil record shows that, about 3.5 billion years ago, microbes began to form. These are tiny, single-cell organisms so small that millions could fit into the eye of a needle. They are the oldest form of life and include protists, a primitive form of algae, as well as fungi and bacteria. Some microbes lived in the water. Other microbes formed sheets that were able to capture layers of sediment, which formed the Earth’s first soil.

Between 2.4 to 3.2 billion years ago, early plants that used photosynthesis began to develop in Earth’s new soil. In photosynthesis, plants take in sunlight, nutrients, and carbon dioxide and then release oxygen into the atmosphere. For a long time, the plants on Earth were so small it would have taken a microscope to see them. But these early plants slowly released oxygen into the air, making Earth able to sustain life.
We can find out about the beginning of many new species by looking at fossils. In an era called the Cambrian Era, which happened around 540 million years ago, animals began to develop hard body parts, like the shells or skeletons of animals like modern-day snails. These hard body parts protected the organisms and allowed them to survive. Around 530 million years ago, vertebrates began to appear. These were the first fish. The fossil record assists scientists in tracing a timeline of animal evolution that goes back millions of years.

Corals began to form around this same time—500 million years ago. Simple, lone organisms formed that were like the corals we have today. Over time, corals appeared, disappeared, and reappeared. They would become extinct because of changes in their environment, only to come back millions of years later. Eventually, “stony” reefs began to form as corals died and clumped together. The reefs eventually teemed with life, but their formation had another effect, too. The erosion of reefs brought nutrients to the coasts, which allowed for life to develop on coastal lands. Several hundred million years ago, plants started growing at the edge of the water and then, gradually, more inland. Once plants grew away from the water, scientists believe that more animals moved away from the water and farther inland.

Some of the fish began to change. Their fins turned into limbs. This helped them to live in water that did not have much oxygen, because they could crawl on land to breathe. Some of them became so well-adapted to living on land that they spent most of their adult lives on land. They would return to the water to lay their eggs. These animals were the first amphibians.
Since the Earth was formed, it has been changing constantly. Life forms have constantly changed, too. Species can evolve and take on new forms, or they can die out because of changes in the environment. Scientists estimate that more than 99% of the species that have ever lived on Earth are extinct. Mammals have become more diverse on Earth because of the environment. Creatures in the ocean could evolve into whales, while creatures that lived in the trees could evolve into primates. As spaces became more open, animals became larger.

So why have so many animals and plants become extinct over time? Huge mass extinctions happened because of climate. The ice age wiped out many species of animals. Another mass extinction event may have happened because of volcanos and asteroid impact. When the asteroids hit Earth, scientists believe it caused global climate change. This impact was not instant—it probably took more than 60,000 years for the Earth to feel the full effects of this global warming, and 10 million years for the planet to recover from it!

Millions of years later, climate and gases in the atmosphere still lead to changes in ecosystems. Global climate change, for example, has changed wooded areas over time into grasslands. This leads to changes in the species of plants and animals in a geographic area on Earth. It can lead to the extinction of a species.