Life on Earth started around 3.8 billion years ago and has since evolved and diversified through the process of natural selection to be adapted to almost every environment possible. There are currently an estimated 1.9 million animals, plants, and other forms of life on Earth.
Life can be found in every nook and cranny/niche of the globe, from the extreme environments of deep sea hydrothermal vents and the freezing conditions of the polar regions to the lush habitats found at the equator.
Looking back through time, by means of the fossil and phylogenetic record, we can see that the Earth has been home to many more species than are alive today. Taking a historical perspective shows that life is constantly evolving, with the success and dominance of different groups waxing and waning over time.
Clip taken from: Charles Darwin and the Tree of Life
With the publication of 'On the Origin of Species by means of Natural Selection' on the 24th November 1859, Charles Darwin not only explained how and why we have the diversity of life we see all around us, but also showed how all life is connected.
Since then we have continued to gather evidence from a range of different disciplines including physiology, biochemistry and DNA analysis. The evidence indicates that all organisms on Earth are genetically related, a genealogical relationship that can be represented as an evolutionary tree known as the Tree of Life.
The Tree of Life illustrates how different species arise from previous species via descent with modification, and that all of life is connected. The diagram above shows the relationship between the major biological groups. The centre represents the last universal ancestor of all life on earth, the outer branches the major biological groups.
The tree is based on research carried out by: David Hillis, Derrick Zwickl and Robin Gutell from the University of Texas. It is based on analysis of small sub-unit rRNA sequences sampled from about 3,000 species from throughout the Tree of Life.
Biologists classify life into a hierarchical family tree at the top of which animals that are similar to each other are grouped together. As you travel down the branches of the tree, so the organisms become more closely related.
At the top of the tree are the kingdoms - the major groups into which all living things are categorised - and at the bottom are individual species. Navigating from one of the major kingdoms listed below, and following through the groups, takes you on a journey through the evolutionary history of life.
The history of life on Earth began about 3.8 billion years ago, during the Archean era, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it's only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals didn't evolve until 200Ma and our own species, Homo sapiens, only 200,000 years ago. So humans have been around for a mere 0.004% of the Earth's history.