It has been known for some time that whales evolved from land dwelling animals. The most recent DNA studies had suggested that their distant ancestral relationships are among the ungulates - a hoofed animal group. Their ancestry however appears to lie far back in this group when the ungulates appear to have been at least partly carnivorous or scavenging.
Further evidence about the steps that the Cetaceans ( whales, dolphins and porpoises) took on their long evolutionary journey from land to water has been revealed by a paper in PLosOne. The paper reports the find of a pregnant fossil Protocetid ( the group that was ancestral to the whales).
The Protocetids still had four legs and show evidence of having been semi aquatic. What is exciting is that in the fossilised pregnant female the foetus presents in a way that can only be interpreted as indicating that this species gave birth on land. This adds further evidence to the theorised evolutionary history of the Cetaceans. A male specimen was also found and comparison of the female and male shows evidence of sexual dimorphism ( Significant differences in body shape or form between the sexes).