Scientists have begun producing chimeras— hybrid creatures that are part human, part animal.

In 2003, Chinese scientists fused human cells with rabbit eggs. The embryos were allowed to develop for several days before the researchers destroyed them to harvest their stem cells.

At the Mayo Clinic last year pigs were created that had human blood flowing through their bodies.

Irving Weissman, from Stanford University in California, has already created mice with brains that are about one percent human.

Later this year he may try to inject human neurons into the brains of embryonic mice, giving the animals 100 percent human brains.

Before being born, the mice would be killed and dissected to see if the architecture of a human brain had formed. If it did, he’d look for traces of human cognitive behavior.

Weissman hopes the experiment leads to a better understanding of how the brain works, which would be useful in treating diseases like Alzheimer or Parkinson. For scientists indeed, more humanlike animals make better research model for testing drugs or growing “spare parts,” such as livers, to transplant into humans.

Not all chimeras are considered troubling: faulty human heart valves can now be replaced with ones taken from cows and pigs; and for years scientists have added human genes to bacteria and farm animals.

What’s problematic here is the mixing of human stem cells with embryonic animals to create new species.

While conceding that these studies would lead to some medical breakthroughs, many believe animals have the right to exist without being tampered with or crossed with another species.

Via Slashdot < National Geographic.