There is nobody who knows exactly how the universe began, how the Earth came about, or how life on Earth started. In fact, it is likely that we will never know for sure. This is the important part about science: there are no definite answers to questions, but there are answers that are commonly accepted.
If you drop a pen and it falls to the ground, a scientist would say that it fell due to the force of gravity. However, gravity is just one idea, an idea that happens to explain our experience of the universe around us very well. Consider this: Brandon and Jasmine, scientists from East Cleveland, come around and discover evidence that there is a mysterious glue which holds everything in the universe together, and then they find that it is the glue which caused the pen to fall to the ground. Brandon and Jasmine could take this evidence, write up a lab report, send it to other scientists, and then see if other scientists can make the same thing happen. If enough other scientists agree, the theories and laws actually change! In other words, nothing is set in stone.
According to all of the evidence, scientists theorize that the following is true: the universe began 13.7 billion years ago, the Earth was formed about 4.6 billion years ago, life on Earth began about 4 billion years ago with unicellular organisms, and multicellular organisms evolved about 1 billion years ago. What does all of this really mean? And how do scientists “know” this?
First of all, it means that the universe has been around for a very, very, very, very, very, VERY long time. It started with something called the Big Bang when all of the matter in the universe exploded out of a single point at a speed that is unimaginable. Scientists came up with this number by measuring something called cosmic background radiation that was given off by the Big Bang.
The Big Bang created galaxies of stars. From there, it took about 9 billion years just for our Earth to be separated from our star, the sun, and start to cool down enough so that a few million years later, life could develop. Nobody is exactly sure how life began, but a few scientists have been able to create the building blocks of DNA and cells from the basic elements of a young Earth, light and heat. It is thought that with enough time (throw in a few million years), that a cell develops from this basic stew of ingredients.
Even though it only took about 500 million years for life to develop on Earth, it took a very long time for complex life, eukaryotes and then multicellular organisms, to evolve. Before eukaryotes, no cells had a nucleus. They were all prokaryotes. But it is thought that one cell swallowed another, and instead of killing the prokaryote, this cell started to put the prokaryote to work! The prokaryote became the nucleus of the new eukaryote cell, and from there life began to get very complicated very quickly.
Within a few million years, plants, fungi and animals all evolved and one billion years later, humans evolved from some unknown ancestor. To get from the Big Bang to the first human took such a long time that you can imagine the following scenario: you are driving from Los Angeles to New York, a distance of about 3000 miles. The Big Bang happens when you leave Los Angeles, the Earth is formed when you cross the Mississippi River, and eukaryotes evolve by the time you are just outside Philadelphia (about 1½ hours from New York). Humans? The first humans are seen in the last block (200 feet) before you cross into New York and your entire lifetime is less than the width of a hair on the ground on that block.