Life on Earth has been changing at various rates since our common ancestor first appeared more than 3.5 billion years ago. To better understand the changes that have taken place, it helps to look for milestones in the history of life on Earth. By grasping how organisms, past and present, have evolved and diversified throughout the history of our planet, we can better appreciate the animals and wildlife that surround us today.
The first life evolved more than 3.5 billion years ago. Scientists estimate that the Earth is some 4.5 billion years old. For nearly the first billion years after the Earth formed, the planet was inhospitable to life. But by about 3.8 billion years ago, the Earth's crust had cooled and the oceans had formed and conditions were more suitable for the formation of life. The first living organism formed from simple molecules present in the Earth's vast oceans between 3.8 and 3.5 billion years ago. This primitive lifeform is know as the common ancestor. The common ancestor is the organism from which all life on Earth, living and extinct, descended.
Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago. A type of organism known as cyanobacteria evolved some 3 billion years ago. Cyanobacteria are capable of photosynthesis, a process by which energy from the sun is used to convert carbon dioxide into organic compounds—they could make their own food. A byproduct of photosynthesis is oxygen and as cyanobacteria persisted, oxygen accumulated in the atmosphere.
Sexual reproduction evolved about 1.2 billion years ago, initiating a rapid increase in the pace of evolution. Sexual reproduction, or sex, is a method of reproduction that combines and mixes traits from two parent organisms in order to give rise to an offspring organism. Offspring inherit traits from both parents. This means that sex results in the creation of genetic variation and thus offers living things a way to change over time—it provides a means of biological evolution.
The Cambrian Explosion is the term given to the time period between 570 and 530 million years ago when most modern groups of animals evolved. The Cambrian Explosion refers to an unprecedented and unsurpassed period of evolutionary innovation in the history of our planet. During the Cambrian Explosion, early organisms evolved into many different, more complex forms. During this time period, nearly all of the basic animal body plans that persist today came into being.
The first back-boned animals, also known as vertebrates, evolved about 525 million years ago during the Cambrian Period. The earliest known vertebrate is thought to be Myllokunmingia, an animal that is thought to have had a skull and a skeleton made of cartilage. Today there are about 57,000 species of vertebrates that account for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates and belong to animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.
The first land vertebrates evolved about 360 million years ago. Prior to about 360 million years ago, the only living things to inhabit terrestrial habitats were plants and invertebrates. Then, a group of fishes know as the lobe-finned fishes evolved the necessary adaptations to make the transition from water to land.
Between 300 and 150 million years ago, the first land vertebrates gave rise to reptiles which in turn gave rise to birds and mammals. The first land vertebrates were amphibious tetrapods that for some time retained close ties with the aquatic habitats they had emerged from. Over the course of their evolution, early land vertebrates evolved adaptations that enabled them to live on land more freely. One such adaptation was the amniotic egg. Today, animal groups including reptiles, birds and mammals represent the descendents of those early amniotes.
The genus Homo first appeared about 2.5 million years ago. Humans are relative newcomers to the evolutionary stage. Humans diverged from chimpanzees about 7 million years ago. About 2.5 million years ago, the first member of the genus Homo evolved, Homo habilis. Our species, Homo sapiens evolved about 500,000 years ago.