READ: Histroy of Life on Earth

Evolution of Major Life Forms

Life on Earth began about 3.5 to 4 billion years ago. The first life forms were single cell organisms, prokaryotic organisms, similar to bacteria. The first multicellular organisms did not appear until about 610 million years ago in the oceans. These of course would be eukaryotic organisms. Some of the first multicellular forms included sponges, brown algae, and slime molds.

Many of the modern types of organisms we know today evolved during the next ten million years in an event called the Cambrian explosion. This sudden burst of evolution may have been triggered by some environmental changes that made the environment more suitable for a wider variety of life forms.

Plants and fungi did not appear until roughly 500 million years ago. They were soon followed by arthropods (insects and spiders). Next came the amphibians about 300 million years ago, followed by mammals around 200 million years ago and birds around 100 million years ago.

Even though large life forms have been very successful on Earth, most of the life forms on Earth today are still prokaryotes – small, single celled organisms. Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; in fact, it is estimated that 99% of the species that have lived on the earth no longer exist.

The basic timeline of Earth is a 4.6 billion year old Earth, with (very approximately):

• about 3.5 - 3.8 billion years of simple cells (prokaryotes)
• 3 billion years of photosynthesis
• 2 billion years of complex cells (eukaryotes)
• 1 billion years of multicellular life
• 600 million years of simple animals
• 570 million years of arthropods (ancestors of insects, arachnids and crustaceans)
• 550 million years of complex animals
• 500 million years of fish and proto-amphibians
• 475 million years of land plants
• 400 million years of insects and seeds
• 360 million years of amphibians
• 300 million years of reptiles
• 200 million years of mammals
• 150 million years of birds
• 130 million years of flowers
• 65 million years since the non-avian dinosaurs died out
• 2.5 million years since the appearance of Homo
• 200,000 years since humans started looking like they do today
• 25,000 years since Neanderthals died out