Evolution and natural selection have played a role in the ever-changing landscape of plants, animals, bacteria and fungi. Although species evolve as they find their niche and adapt to new opportunities, some animals have remained relatively unchanged over the course of history.
These animals are known as living fossils. Compared to the animals on this list, humans are relative newcomers to this planet. Homo sapiens emerged out of Africa a mere 200,000 years ago.
Many living fossils are considerably older than humans and other mammals; some have even outlasted the dinosaurs. In this slideshow, take an up-close look at animals that have persevered virtually unchanged through the ages and continue to thrive today.
Horseshoe Crab
The horseshoe crab could hold the distinction of being the oldest animal species still in existence.
Dating back to the Paleozoic era, the horseshoe crab existed on Earth before the dinosaurs and soldiered on through several mass extinction events.
In 2008, a horseshoe crab fossil, the oldest in existence found so far, dated back to around 445 million years ago, according to a report by LiveScience.
Triops Cancriformis
The tadpole shrimp, Triops cancriformis, is another contender for the title of oldest living animal species. This shrimp is related to the horseshoe crab so its longevity should come as no surprise.
According to a report by The Telegraph, the tadpole shrimp as it appears today is virtually identical to a fossil of a specimen that lived some 200 million years ago just as dinosaurs rose to prominence.
Despite the animal's remarkable endurance, the tadpole shrimp is currently listed as an endangered species.
Coelacanth
Once thought to be extinct in the same event that killed off the dinosaurs some 65 million years ago, the coelacanth is a lobe-finned fish that sparked a debate over whether this species represented a missing link between aquatic animals and four-legged terrestrial creatures, according to National Geographic.
The animal was rediscovered in 1938 and only two species of coelacanth still exist today. In 2007, a fossilized coelacanth fin was found dating back roughly 400 million years.
Common Snapping Turtle
Snapping turtles as we know them first walked the earth some 40 million years ago, but they have been virtually unchanged over the past 215 million years of their evolution, according to Tortoise Trust.
Although not among the most endangered tortoises and turtles according to the Turtle Conservation Coalition, the snapping turtle is listed as threatened.
Nautilus
The nautilus is the most primitive cephalopod in existence, a group that includes the most complex squid and octopus.
Dating back to more than half a billion years ago, the nautilus reached the high point in its evolution during the Paleozoic era about 505 million to 408 million years ago.
Several species of nautilus still survive today -- relatively unchanged from their ancestral counterparts.
Goblin Shark
Goblin sharks are rare, deep-sea dwellers with a unique elongated nose that distinguishes them from other sharks. They're also ancient, and are between 112 million to 124 million years old as a species.
Around 2,000 different species of fossil sharks have been discovered, according to the ReefQuest Centre for Shark Research. The earliest sharks predate the dinosaurs by more than 200 million years.
Hagfish
Hagfish may have had to endure a less-than-flattering name since scientists first described them in the 18th century. However, these famously ugly marine animals have existed for about half a billion years.
The hagfish also represents an important evolutionary step in the development of vision. These ancient fish may have been among the earliest animals to evolve more complex, camera-like eyes as opposed to the strictly photosensitive vision possessed by more primitive species. As such, the hagfish represents a kind of missing link in the evolution of the eye.
Mouse Deer
Compared to other animals on this list, the mouse deer, better known as a chevrotain, is a relative newcomer. For a large mammal, however, it's relatively old.
This animal is among the only survivors of a group of hoofed mammals that lived some 35 million years ago.
Oarfish
The oarfish has a flattened, snake-like body that grows up to almost 10m long. It has a bright red dorsal fin and a crest of long stiff rods, or fin-rays, on the top of its head.
This strange and harmless fish is seen occasionally at the surface and may even be washed on to the shore. Because we see it so rarely, little is known about the oarfish's lifestyle. Its stomach contents reveal that it feeds mainly on tiny animals that drift through the water.
Frilled Shark
The frilled shark has been called everything from a “sea serpent” to a real-life “Loch Ness Monster” over the years in places where it lives, such as southeast Australia, New Zealand, Southeast Asia, West Africa, Chile and the Caribbean.
When open, the frilled shark’s mouth reveals 300 trident-shaped teeth aligned in 25 rows. Aside from its unmistakable toothiness, the mouth looks larger than that of other sharks because its jaws terminate at the back of the fish’s head instead of underneath the skull. The head appears to be all mouth, capped off at the throat region with six frilled gills, hence its name.
Horseshoe Crab
The horseshoe crab could hold the distinction of being the oldest animal species still in existence.
Dating back to the Paleozoic era, the horseshoe crab existed on Earth before the dinosaurs and soldiered on through several mass extinction events.
In 2008, a horseshoe crab fossil, the oldest in existence found so far, dated back to around 445 million years ago, according to a report by LiveScience.
Triops Cancriformis
The tadpole shrimp, Triops cancriformis, is another contender for the title of oldest living animal species. This shrimp is related to the horseshoe crab so its longevity should come as no surprise.
According to a report by The Telegraph, the tadpole shrimp as it appears today is virtually identical to a fossil of a specimen that lived some 200 million years ago just as dinosaurs rose to prominence.
Despite the animal's remarkable endurance, the tadpole shrimp is currently listed as an endangered species.
Coelacanth
Once thought to be extinct in the same event that killed off the dinosaurs some 65 million years ago, the coelacanth is a lobe-finned fish that sparked a debate over whether this species represented a missing link between aquatic animals and four-legged terrestrial creatures, according to National Geographic.
The animal was rediscovered in 1938 and only two species of coelacanth still exist today. In 2007, a fossilized coelacanth fin was found dating back roughly 400 million years.
Common Snapping Turtle
Snapping turtles as we know them first walked the earth some 40 million years ago, but they have been virtually unchanged over the past 215 million years of their evolution, according to Tortoise Trust.
Although not among the most endangered tortoises and turtles according to the Turtle Conservation Coalition, the snapping turtle is listed as threatened.
Nautilus
The nautilus is the most primitive cephalopod in existence, a group that includes the most complex squid and octopus.
Dating back to more than half a billion years ago, the nautilus reached the high point in its evolution during the Paleozoic era about 505 million to 408 million years ago.
Several species of nautilus still survive today -- relatively unchanged from their ancestral counterparts.
Goblin Shark
Goblin sharks are rare, deep-sea dwellers with a unique elongated nose that distinguishes them from other sharks. They're also ancient, and are between 112 million to 124 million years old as a species.
Around 2,000 different species of fossil sharks have been discovered, according to the ReefQuest Centre for Shark Research. The earliest sharks predate the dinosaurs by more than 200 million years.
Hagfish
Hagfish may have had to endure a less-than-flattering name since scientists first described them in the 18th century. However, these famously ugly marine animals have existed for about half a billion years.
The hagfish also represents an important evolutionary step in the development of vision. These ancient fish may have been among the earliest animals to evolve more complex, camera-like eyes as opposed to the strictly photosensitive vision possessed by more primitive species. As such, the hagfish represents a kind of missing link in the evolution of the eye.
Mouse Deer
Compared to other animals on this list, the mouse deer, better known as a chevrotain, is a relative newcomer. For a large mammal, however, it's relatively old.
This animal is among the only survivors of a group of hoofed mammals that lived some 35 million years ago.
Oarfish
The oarfish has a flattened, snake-like body that grows up to almost 10m long. It has a bright red dorsal fin and a crest of long stiff rods, or fin-rays, on the top of its head.
This strange and harmless fish is seen occasionally at the surface and may even be washed on to the shore. Because we see it so rarely, little is known about the oarfish's lifestyle. Its stomach contents reveal that it feeds mainly on tiny animals that drift through the water.
Frilled Shark
The frilled shark has been called everything from a “sea serpent” to a real-life “Loch Ness Monster” over the years in places where it lives, such as southeast Australia, New Zealand, Southeast Asia, West Africa, Chile and the Caribbean.
When open, the frilled shark’s mouth reveals 300 trident-shaped teeth aligned in 25 rows. Aside from its unmistakable toothiness, the mouth looks larger than that of other sharks because its jaws terminate at the back of the fish’s head instead of underneath the skull. The head appears to be all mouth, capped off at the throat region with six frilled gills, hence its name.