Earth first formed about 4.6 billion years ago. When the planet was very young, life would have been impossible—the ground was blisteringly hot and there was no water in sight. So how did life begin?
A sea of molten rock covered the newly formed Earth. In time, this cooled to solid rock, but volcanoes continued to spew out floods of lava. The volcanoes also released gases from deep inside the planet, forming Earth’s atmosphere, though the air at first was poisonous.
COMETS AND ASTEROIDS
For millions of years, Earth’s surface was bombarded by comets, asteroids, and even small planets. The collisions tore open the planet’s newly formed crust, releasing more floods of lava. But they also delivered water.
As the young Earth slowly cooled, so did its atmosphere. Scalding steam released by volcanoes condensed to form liquid water that fell as rain, producing a downpour that lasted as long as a million years. Comets and
asteroids brought yet more water. All the water pooled on the surface to form vast oceans.
A watery beginning
Many scientists think life began about 3.8 billion years ago in the deep sea, which was safer than Earth’s deadly surface. The first life-forms might have lived around hot volcanic vents, feeding off energy-rich chemicals dissolved in the boiling water. Special kinds of
bacteria thrive in these scalding habitats even today.
Life in hot water
At Grand Prismatic Spring in Yellowstone National Park, bacteria thrive in water too hot for any other organism to bear.
The first life-form was not a whole organism or even a cell—it was just a
molecule that could make copies of itself. This is what DNA does today. DNA can’t copy itself outside cells, so the first living molecule must have been something different. Later on, it evolved into DNA.
The age of bacteria
Soon after life began, the self-copying molecules built cells around themselves and became bacteria. Bacteria were the only forms of life on Earth for the next 3 billion years, a vast span of time.
A true survivor – Stromatolites
Some of the oldest evidence of life on Earth comes from stromatolites. These are rocklike mounds formed by colonies of bacteria. Fossil stromatolites date back to 3.5 billion years ago. The bacteria in stromatolites live like plants, using the Sun’s energy to make food and in doing so releasing oxygen. Billions of years ago, they made enough oxygen to transform Earth’s air, paving the way for air-breathing animals to evolve.