As usual, In Our Time today was engrossing and revealing – and especially interesting after our recent visit to Down House and yesterday’s unveiling of  Ida, humanity’s long-lost relation. Today it was a discussion between Steve Jones, Professor of Genetics at University College London, Eleanor Weston, a mammalian palaeontologist at the Natural History Museum in London and Bill Amos, Professor of Evolutionary Genetics at Cambridge University.

Melvyn Bragg opened the discussion as follows:

‘Of all the whales in literature, among the most famous is Moby Dick described by Herman Melville:

Melville’s story is one of drama and grim portent, but far more extraordinary is the story of the whale itself. For the manner in which the whale has evolved is among the finest exemplars of the changes evolution can bring to bear upon life on Earth.’

“Moby Dick moved on, still withholding from sight the full terrors of his submerged trunk, entirely hiding the wrenched hideousness of his jaw. But soon the fore part of him slowly rose from the water; and warningly waving his bannered flukes in the air, the grand god revealed himself, sounded and went out of sight”.

It’s the story of an ancient land mammal making its way back to the sea, becoming the forerunner of today’s whales. In doing so, it lost its legs, and all of its vital systems became adapted to a marine existence — the reverse of what happened millions of years previously, when the first animals crawled out of the sea onto land.

Humpback whale

Some details remain fuzzy and under investigation. But we know for certain that this back-to-the-water evolution did occur, thanks to a profusion of intermediate fossils that have been uncovered over the past two decades. In 1978, palaeontologist Phil Gingerich discovered a 52-million-year-old skull in Pakistan that resembled fossils of creodonts — wolf-sized carnivores that lived between 60 and 37 million years ago, in the early Eocene epoch. But the skull also had characteristics in common with the Archaeocetes, the oldest known whales. The new bones, dubbed Pakicetus, proved to have key features that were transitional between terrestrial mammals and the earliest true whales. One of the most interesting was the ear region of the skull. In whales, it is extensively modified for directional hearing underwater. In Pakicetus, the ear region is intermediate between that of terrestrial and fully aquatic animals.

Another, slightly more recent form, called Ambulocetus, was an amphibious animal. Its forelimbs were equipped with fingers and small hooves. The hind feet of Ambulocetus, however, were clearly adapted for swimming. Functional analysis of its skeleton shows that it could get around effectively on land and could swim by pushing back with its hind feet and undulating its tail, as otters do today.

Rhodocetus shows evidence of an increasingly marine lifestyle. Its neck vertebrae are shorter, giving it a less flexible, more stable neck — an adaptation for swimming also seen in other aquatic animals such as sea cows, and in an extreme form in modern whales. The ear region of its skull is more specialized for underwater hearing. And its legs are disengaged from its pelvis, symbolizing the severance of the connection to land locomotion.

By 40 million years ago, Basilosaurus — clearly an animal fully adapted to an aquatic environment — was swimming the ancient seas, propelled by its sturdy flippers and long, flexible body. Yet Basilosaurus still retained small, weak hind legs — baggage from its evolutionary past — even though it could not walk on land.

Recently, fossils have been discovered of an immediate ancestor to the whale. It is Indoyhus, “a small deer-like animal that waded in lagoons and munched on vegetation.” It doesn’t look much like what we know as whales, and it’s only about the size of a domestic cat, but it shares characteristics with modern whales that other fossils don’t, such as bone structure. Whale evolution is thought to have begun with creatures like Indohyus becoming more adapted to a watery environment to avoid land-based predators. The animal’s heavy bones would have made Indohyus a slow beast on land, but in the water, they would help it stay on the bottom, where it could forage and hide.

None of these animals is necessarily a direct ancestor of the whales we know today; they may be side branches of the family tree. But the important thing is that each fossil whale shares new, whale-like features with the whales we know today, and in the fossil record, we can observe the gradual accumulation of these aquatic adaptations in the lineage that led to modern whales.

As evolutionary biologist Neil Shubin points out, “In one sense, evolution didn’t invent anything new with whales. It was just tinkering with land mammals. It’s using the old to make the new.”

Whale evolution

Whale Evolution: by Nature Video

The marine mammals known as cetaceans originated about 50 million years ago in south Asia, but their terrestrial ancestor is something of a mystery. Hans Thewissen and colleagues now provide the missing Eocene piece of the jigsaw.

Ken Miller on Whale Evolution and the failure of Intelligent Design

Dr. Ken Miller of Brown University talks about the reality of transitional fossils within the fossil record and how they help to support the theory of Evolution.



3 thoughts on “How whales evolved

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