The origins of hair date back 310 million to 330 million years ago to the last common ancestor of mammals, birds and lizards, according to a new study that discovered genes associated with hair production in living green anole lizards and chickens.

While lizards and chickens are not hairy, their claws contain proteins nearly identical to those found in the human hair shaft, fingernails and toenails, on the surface of the tongue and within the thymus gland.

Since the last common ancestor of mammals, birds and lizards lived before the first true dinosaurs emerged, both dinosaurs and humans appear to have inherited the genes responsible for human hair and animal claws.

The researchers additionally found that keratin proteins in lizards and humans contain a high content of an amino acid called cysteine. Tracing the history of this acid, the scientists found it didn't just suddenly arise by chance during mammalian evolution, but instead was inherited from the ancient common ancestor.

The particulars of this important animal relative remain unknown, but scientists can make some educated guesses.

"Probably it was more similar to a lizard than to modern mammals or birds," Eckhart said. "It is likely that this ancestor had claws, which it may have used for climbing."

The first mammal emerged sometime later, likely between 160 million and 220 million years ago. Since the researchers believe hair keratins evolved earlier than hair itself, some of the first mammals could have sported a warm, furry coat.

Radiometric dating involves the use of isotope series, such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years. Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age.

Newts are members of the salamander family that breed in water. They are the only vertebrates that can regenerate large parts of themselves, growing new limbs, spinal cords, hearts, jaws, tails and even eyes.

As the damaged part heals, the cells reverse their original function and turn back into an undifferentiated lump called a blastema (from the Greek blastos for "bud"), from which the replacement limb or tissue grows. How the cells know what to grow isn't understood, but salamanders are being studied closely to see whether or not human tissue could be stimulated to regenerate.