Bacteria precede multicellular life by 3 billion years. Obviously, there must be advantages to being a bacterium.
It is likely that all life on earth is descended from a single bacterium that lived eons ago. They call this creature the Last Universal Common Ancestor.
Trilobites flourished for 300 million years. They were the first of the arthropods, and gave rise to countless forms of such creatures.
Ants are probably the most successful animals ever to live: they can be found almost everywhere, and are in no danger of extinction. They are a magnificently adaptable and hardy form of life.
Some dinosaurs escaped the asteroid Chicxulub by growing feathers to keep warm. They became birds.
All land animals are probably descended from lobe finned fish, which could breathe air and learned how to walk on land.
Sponges are the oldest lineage of animals that still thrives.
There is some evidence to suggest that ancient Krakens may have been intelligent; a fossilized lair of one was found, with the bones of prey animals neatly arranged.
Lichens are actually a symbiotic relationship between an algae and a fungus. They can live almost anywhere.
Bacteria have been found in very inhospitable places. Some people theorize they may live in the mantle as well as the deep crust. They could probably live on Mars and other bodies, but it is unknown whether they could have developed there independently.
There are tiny ecosystems that never see the sun; they are based on bacteria that feed on chemicals. These ecosystems exist in caves or the seafloor. Many eons ago, similar bacteria were the only form of life on earth, before photosynthesis was invented.
The bottom of the sea is covered with a sludge made of bacteria and rotted organic material from above. In many years, this sludge will get compacted into petroleum.
Amphibians used to rule the land and attained fearsome proportions. They were later supplanted by the more adaptable reptiles.
Jellyfish are the simplest animals, but surprisingly resilient. They have been around far longer than almost anything else.
Most of the life on earth is marine; by mass, krill and plankton probably make up well more than half of all life.