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Scientists have mapped the genetic composition of a marsupial mammal, the South American grey, short-tailed opossum, gaining insight into the role of "junk DNA" in human evolution and into immune systems.

Because this opossum develops melanoma skin cancer much as people do and its newborns can regenerate a severed spinal cord, scientists hope studying its genome can boost research into treating human skin cancer and neurological ailments.

In research published on Wednesday, the furry creature - twice as big as a mouse and with a prehensile tail - became the first marsupial to have its DNA decoded.

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The research, appearing in the journals Nature and Genome Research, was led at the Broad Institute of Harvard University and Massachusetts Institute of Technology and included scientists from around the world.

Marsupials are closely related to placental mammals, the group that includes humans, but their evolutionary lines diverged 180 million years ago during the dinosaur age.

Using the opossum as a comparison helped identify genetic elements present in placental mammals but absent in marsupials, helping to fill a hole in the understanding of how mammalian genomes have evolved over tens of millions of years and giving a new look at the evolutionary origins of the human genome.

One-fifth of the human genome's key functional elements arose after the divergence from marsupials, the research found. Most of these innovations occurred not in protein-coding genes but in areas of the genome that do not contain genes and until recently had been derided as junk DNA, they found.

The findings indicated that mammals evolved less by inventing new kinds of proteins and more by adjusting the molecular controls over when and where proteins are made.

Broad Institute chief Eric Lander said it was surprising to learn that many of these new regulatory controls were coming from "jumping genes" that have bounced around chromosomes for more than a billion years.

These "jumping genes," called transposons, also are in areas once believed to be junk DNA.

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But Mr. Lander said the research demonstrated that jumping genes, rather than being a sort of genetic parasite that just copied themselves, instead played a positive evolutionary role by spreading genetic innovations.

Three types of mammal

There are three types of mammals - animals that have hair, produce milk to nourish babies and have high metabolic rates.

Most common are placental mammals such as rodents, bats, dogs, cats, whales and people that bear live offspring that are nourished before birth in the mother's uterus.

Marsupials give live birth, without the long gestation of placental mammals, to tiny babies at a very early stage of development that climb from the birth canal to the mother's nipples, where they latch on for weeks or months. Examples include the kangaroo, wallaby and koala.

The most primitive mammals are monotremes, which lay eggs rather than having live birth. They include only the duck-billed platypus and the echidna, also known as the spiny anteater. Scientists are studying the platypus to try to decode the first monotreme genome.

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Katherine Belov of the University of Sydney in Australia said the research showed the complex mammalian immune system - the body's natural defences - arose before the two lineages separated.

"By aligning 1,500 human immune genes to the opossum genome, we discovered that the opossum genome is very similar to that of humans," Ms. Belov said by e-mail.

"For a long time, people have thought that the marsupial immune (system) is primitive and retarded. We have overturned this view. Marsupials are capable of complex immune responses which are on par with those of humans."

Marsupials - born without a functioning immune system - develop one while growing outside the mother's body.

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