Earth Discovery
Tis is how we found out about Earth and Life.
Scientists worked this out rather like detectives reconstructing an event that nobody witnessed.
Earth formed about 4.54 billion years ago Scientists discovered its great age by studying rocks, meteorites and the radioactive elements inside them.
Certain radioactive atoms decay at predictable rates, effectively providing a natural clock. Meteorites are particularly useful because they formed at about the same time as the Solar System.
At first, the young Earth was extremely hot. Material collided and joined together as the planets formed around the young Sun. Eventually Earth's surface cooled enough for a crust and liquid water. Ancient minerals called zircons, some more than four billion years old, contain chemical clues suggesting that liquid water existed remarkably early.
For life's history, scientists have another enormous collection of clues: fossils.
The oldest evidence concerns microscopic organisms rather than animals. For billions of years, Earth was essentially a microbial world.
Some microorganisms evolved photosynthesis, releasing oxygen. Eventually enough accumulated to transform the oceans and atmosphere.
By around 600 million years ago, strange soft-bodied organisms lived in the oceans.
Then, during the Cambrian Period beginning about 539 million years ago, animal life diversified dramatically. Fossils preserve creatures with shells, legs, eyes and increasingly complex bodies.
Much later, vertebrates evolved from ancient aquatic ancestors. Some eventually ventured onto land.
Over hundreds of millions of years their descendants diversified into amphibians, reptiles and eventually many other groups.
Dinosaurs appeared roughly 230 million years ago.
Mammals existed alongside them, and for much of the dinosaur era many mammals remained relatively small. Imagine a Cretaceous night about 85 million years ago: enormous dinosaurs might be outside, while little furry mammals scurried through vegetation, climbed trees or hid in burrows.
Then came a remarkable clue in the rocks. Around the world scientists found a thin layer unusually rich in iridium, an element associated with meteorites. They also discovered the enormous Chicxulub impact crater in Mexico.
Together with many other lines of evidence, these showed that a giant asteroid struck Earth about 66 million years ago, contributing to the mass extinction that eliminated all non-avian dinosaurs.
Mammals survived.
Over millions of years they diversified into many forms—whales, bats, elephants, rodents, primates and countless extinct species.
Our own story occupies only the tiniest final portion. Early primates appeared tens of millions of years ago. Much later in Africa, one branch of apes produced various hominins that increasingly walked upright.
There wasn't a straight ladder from ape to human; human evolution was a branching family tree containing many species.
Eventually our species, Homo sapiens, appeared in Africa roughly 300,000 years ago.
And how do we know that? Fossilised bones and footprints reveal anatomy and movement. Stone tools reveal behaviour.
Layers of rock establish sequences in time. Radiometric dating provides dates. Ancient DNA reveals relationships between populations and species. Genetics in living humans provides another record of our ancestry.
So scientists didn't discover Earth's history from one fossil or one clever idea. Rocks, fossils, chemistry, physics, geology, astronomy and DNA independently provide pieces of the same enormous story.
If Earth's entire 4.54-billion-year history were squeezed into a single 24-hour day, dinosaurs wouldn't appear until about 10:47 p.m., and Homo sapiens would arrive only in roughly the last six seconds before midnight.
We really are extremely recent arrivals.