Age: 3.5 billion years old
The oldest known cyanobacteria fossils were found on Archaean rocks of western Australia. Cyanobacteria, or blue-green algae, is a type of bacteria that uses photosynthesis to create energy. It is believed that this played a role in oxidizing the earth's atmosphere, making the planet more suitable for life as we know it.
Age: 580 million years old
Sponges are from an ancient animal group whose lineage can be traced back to the beginnings of animal life. Fossils of glass sponges have been found in rocks in Australia, China, and Mongolia. Although about 90% of modern sponges are demosponges, fossilized remains of this type are less common than other types because their skeletons are composed of relatively soft spongin that does not fossilize well.
Age: 505 million years old
Jellyfish belong to the group of animals called Cnidaria or Coelenterata. This group includes corals, hydras, jellyfish, Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans. They are hard to fossilize, as they're made of mostly water, but fossils suggest they are even older than previously thought.
Age: 450 million years old
Horseshoe crabs are considered "living fossils." The earliest horseshoe crab fossils are dated to the Ordovician period. These marine arthropods live primarily near shallow ocean waters with soft, sandy or muddy bottoms. Their populations have been in decline due to habitat destruction and over harvesting.
Age: 270 million years old
The Ginkgo tree is the only living representative of the order Ginkgoales, a group of gymnosperms dating back to 270 million years ago in the Permian period. Due to geological cataclysms, only three or four species were left in the Tertiary period (65 million years ago). The extinction of the dinosaurs as potential seed dispersers of the tree's large seeds may also have influenced this decline, which is in line with the fossil records.
Age: 200 million years old
This little guy has the distinction of being the oldest living species on earth that has existed UNCHANGED for 200 million years. In other words, he may not have been around as long as some of the creatures above him, but today he's is still virtually indistinguishable from his 200 million year old fossil.
Age: 200 million years old
Sturgeon and related paddlefish have undergone remarkably little morphological change, indicating that their evolution has been slow and earning them informal status of "living fossils." This is explained, in part, by their long inter-generation time, tolerance for wide ranges of water temperature and salinity, lack of predators due to size, and the abundance of prey items in their benthic environment.