Evolutionary biologists face a challenge that's a lot like a challenge of studying ancient human history: to retrieve vanished connections. The people who live in remote Polynesia presumably didn't sprout from the island soil like trees--they must have come from somewhere. Tracing their connection to ancestors elsewhere hasn't been easy, in part because the islands are surrounded by hundreds of miles of open ocean. It hasn't been impossible though: studies on their culture, language, and DNA all suggest that the Polynesians originally embarked from southeast Asia. We may never be able to retrieve the full flow of history that carried people thousands of miles to the middle of the Pacific, but we can know some things about it, and we can rest assured that some things are definitely not true (such as the sprouting-from-the-ground theory).
Whales are a lot like Polynesians. All living species of whales look a lot like each other, and not very much like any other animals. They all have horizontal tail flukes, blowholes, and smooth skin free of scales or fur. Darwin argued that whales were not simply created in the oceans in their current form, but instead descended from land mammals which had adapted to life in the ocean. He pointed out that whales share a number of traits with land mammals, such as milk and a placenta. Their blowhole connects to a set of lungs very much like those of land mammals and nothing like the gills of fish.
