Hummingbirds: Small in size, extreme in life and complicated in evolution

For such small creatures, hummingbirds certainly have racked up an outsized list of unique claims to fame, which now includes a complicated evolutionary history.

They are the smallest birds and the smallest warm-blooded animals on Earth. They have the fastest heart and the fastest metabolism of any vertebrate. They are the only birds that can fly backward. And scientists reported on Thursday that they also have a complicated evolutionary history.