Scientists had a hunch back in the year 1900 that forest elephants were a separate species from bush elephants. By studying the DNA of mammoths and modern African elephants, paleontologists have rediscovered a species of African elephant once ruled out by appearance alone-- and it's been right in front of us the whole time, alive!

LA Times - African elephants are actually two species (press release)

Rohland, N., D. Reich, S. Mallick, M. Meyer, R.E. Green, N.J. Georgiadis, A.L. Roca, and M. Hofreiter. (2010). Genomic DNA sequences from mastodon and woolly mammoth reveal deep speciation of forest and savanna elephants. PLoS Biology. 8[12]: e1000564. doi:10.1371/journal.pbio.1000564