The germs that help cattle eat grass and gorillas gorge on leaves may have been the secret weapon that let mammals populate the planet, researchers reported on Thursday. Two separate teams of researchers reported on the bacteria living in and on the bodies of humans and other animals, and found they are surprisingly well-adapted to their hosts -- so well that they may have helped different species evolve. "We have evolved together with our bacteria," Dr. Julie Segre of the National Human Genome Research Institute said in a telephone interview.