Evolution of cetaceans

The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air; they have finger bones in their fins; they nurse their young; and, despite their fully aquatic life style, they retained many skeletal features from their terrestrial ancestors. Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

The two modern parvorders of cetaceans – Mysticeti (baleen whales) and (toothed whales) – are thought to have separated from each other, and the archaeocetes, around 28-33 million years ago in a second cetacean radiation, the first occurring with the archaeocetes. The adaptation of echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales. The presence of baleen in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence (and subsequent increase in filter feeding).

Modern-day cetacean evolution is largely affected by local culture and social network. Tool-use for foraging in certain societies affects the diet, and allows them to access more varieties of food. This behavior is passed down from mother-to-child, as in the bottlenose dolphins residing in Shark Bay, where many use sponges to protect their beak while foraging. Social bonds between dolphins in a pod can affect overall fitness in a habitat, decreasing or increasing the survival chances for any particular individual in that pod. Social bonds between different species can sometimes cause interbreeding, which always results in infertile hybrids when it occurs.

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