The oldest living animal species on Earth cannot be known for sure, because not every animal species or fossil has yet been discovered, but our current best guess is the horseshoe crab, which has remained pretty much unchanged since the Ordovician period, 445 million years ago. To put this in perspective, multicellular animals only appear in the fossil record about 600 million years ago, and the typical duration of an animal species is just a few million years. For instance, Tyrannosaurus rex lived for only about three million years. In contrast, the horseshoe crab has existed for about 74% of the time that animals in general have.

Because of its status as one of the oldest living animals, the horseshoe crab (which is actually more closely related to spiders, ticks, and scorpions than crabs) has been dubbed a living fossil. Horseshoe crabs are thought to descend from eurypterids (sea scorpions), which are even older (yet extinct), living as far back as 510 million years ago. As far as we know, no animals from the Cambrian (542 - 488 million years ago) or Ediacaran (635 - 542 million years ago) periods is alive today, although there are numerous fossils.

The horseshoe crab is notable for its simple but effective immune system. When invaded by a foreign organism, such as bacteria, the blood in the local area of the invasion immediately clots into a gel-like substance. This substance, called Limulus Amebocyte Lysate, has been used to test for the presence of bacteria on a surface, exceeding the capabilities of purely artificial detectors. When bacteria is present, it immediately clots. This excellent immune system has surely contributed to the horseshoe crab becoming the oldest living animal.

There have been groups of bacteria, such as cyanobacteria, that have been around for not only hundreds of millions of years, but actually several billion, far exceeding the age of the oldest living animal, but bacteria are not animals. The oldest living plant is thought to be gingko biloba, with fossils found dated to 270 million years ago. The long duration of this plant species is thought to be due to a combination of insect-resistant wood and the ability to form aerial roots and sprouts. A fish, the Coelacanth, is a runner-up for the oldest living animal, appearing in the fossil record 410 million years ago.