Aug. 30, 2011 --
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. We begin with the platypus, an unusual egg-laying animal with fur, a bill and a venomous bite. Charles Darwin himself coined the term "living fossil" while observing the platypus. Native to eastern Australia, the animal is the only surviving example of its family, Ornithorhynchidae. This group of animals is believed to have split from mammals some 166 million years ago.
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.
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.
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.
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.
The more than 20 species of alligators and crocodiles living today have evolved beyond their more primitive ancestors. But the basic physical design of these reptiles has remained essentially the same for the past 320 million years or so. Alligators and crocodiles share a common ancestry, though the two groups separated from each other some 60 million years ago.
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 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.
The cockroach is famous for being a survivor. These insects can survive for weeks without their heads and even withstand the fallout following a nuclear blast. Cockroaches are also an especially long-surviving animal. Roaches have thrived on Earth for some 320 million years, with an estimated 5 million to 10 million individual species ranging in shape, size and habitat. This photo shows Blaberus giganteus, one of the largest species of cockroach on Earth.
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.
The gene that most likely determines the sex of the platypus and echidna has been identified by Australian and Swiss researchers.
The study also shows that the Y chromosome, contrary to previous assumptions, carries genes that are important to the basic viability of male mammals, says geneticist Dr Paul Waters from the University of New South Wales.
Although the Y chromosome is known to be important in sex determination, little is known about the function and evolution of its genes, says Waters.
He says this is because it has so many repetitive and palindromic sequences, which make it hard to reconstruct the true sequences of its genes from fragments of sequenced DNA.
Monotremes (the platypus and the echidna), whose males have 5 X chromosomes and 5 Y chromosomes, are especially challenging.
"No one had really characterised any Y chromosomes in platypus before because they've got quite a complex sex chromosome system," says Waters.
Waters and colleagues from the University of Adelaide and the University of Lausanne now report on their new analysis of male and female DNA from 15 representative mammals, including human, elephants, marsupials and monotremes.
The study, reported recently in the journal Nature, is the largest of its kind, and relied on a rapid new sequencing technique.
For each species, the researchers identified Y chromosome genes by looking for those DNA sequences that were specific to males.
By using a molecular clock, which combines fossil evidence and rates of change in DNA sequences, the researchers were also able to work out when specific genes evolved.
Waters says the process uncovered for the first time a gene, called AMH [for Anti-Müllerian hormone], on the oldest of the platypus Y chromosomes that appears to determine if an animal becomes male.
"If an animal has that gene it will act as a master switch to turn on testis development," he says.
"That was quite a big finding because we had no idea what might be causing male platypus to develop as males."
Waters says short of knocking out the gene in platypus and finding out how it affects their sex, this is the "best evidence to date" on what determines if an animal becomes male.
Unlike monotremes, male marsupials, like kangaroos, and placental mammals, like humans, have only one X and one Y chromosome, and a different sex-determining gene, called SRY.
Waters and colleagues worked out that both this Y chromosome system and the monotreme system evolved around 180 million years ago, but arose independently of other.
"It's a case of convergent evolution," he says.
The researchers are not sure what determined sex prior to the evolution of these genes.
"It could have been an environmental cue such as temperature or could have been a different genetic sex determining system that doesn't exist today," says Waters.
Waters says Y chromosomes started their existence as non-sex chromosomes but over time lost a number of genes.
"As the Y chromosome evolved, it withered away, losing most of the 1000 genes that are found on today's X chromosome."
He says because females don't have a Y chromosome it was believed that over all genes on the Y chromosome could not be too important.
"It was hypothesised that the Y chromosome can't hold anything that's critical to life."
But it turns out this is not the case.
The new study has found that while some of the genes preserved on the Y chromosome evolved for male-specific functions, such as testis development or sperm production, in most species, most Y-specific genes are actually important for the male's basic viability.
They include genes involved in regulation of protein production, which require two copies to be functional. In males, this means having one copy on the X chromosome and one on the Y.