Evidence for Evolution: The Origin of Whales

Ryan M Bebej

University of Michigan

February 10, 2008

Evidence for Evolution: The Origin of Whales

What exactly is meant by evolution?

Where does evidence for evolution come from?

What fossil evidence is there for the evolution of whales from land mammals?

What is evolution?

“In biology, nothing makes sense except in the light of evolution.”

“No educated person any longer questions the validity of the so-called theory of evolution, which we now know to be a simple fact.”

- Ernst Mayr (1904-2005)

What is evolution?

• Biological evolution: changes in the gene frequencies of populations over time– Microevolution: changes in the genetic make-

up of populations of species over short time frames due to natural selection, genetic drift, & mutation; can result in speciation

– Macroevolution: “evolution above the species level”; concerned with origin of higher-level groups & broad-scale patterns of change such as adaptive radiations & origin of novelties

What is evolution?

• Evolutionism: a naturalistic worldview that uses evolutionary biology to support the claim that there is no God & that there is no purpose to human existence

This is philosophical & not scientific, yet it has become thoroughly entangled with the scientific components of evolution…

What is evolution?

• Evolution as mechanism: the process that renders change in lineages through time (natural selection, genetic drift, mutation)

• Evolution as history: the fact that life has changed through time– Common descent: all forms of life are linked in

a family tree & share common ancestry– This provides a robust rationale for explaining

many aspects of embryology, comparative anatomy, biogeography, paleontology, etc.

What is evolution?

“Evolution is both a fact and a theory. It is a fact that evolutionary change took place. And evolution is also a theory that seeks to explain the detailed mechanism behind that change.”

- Kenneth R. Miller, Brown UniversityAuthor of Finding Darwin’s God: A Scientist's Search for Common Ground Between God and Evolution

Evidence for EvolutionAnatomy Development

Genetics Fossils

Beluga Humpback whale

Orca

Blue whale

Order Cetacea

Whales are mammals…

…but they are very different from terrestrial mammals!

1859: Charles Darwin“In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.”

- from The Origin of Species (ch.6)

…was ridiculed and omitted much of this from the 2nd edition

1936: Remington Kellogg

• Seminal “Review of the Archaeoceti”– North American & Egyptian whales– All clearly fully aquatic

50 cm

50 cm

Zygorhiza kochii

Basilosaurus cetoides

Hind limb vestiges

Gray whale

1945: George Gaylord Simpson

“Because of their perfected adaptation to a completely aquatic life, with all its attendant conditions of respiration, circulation, dentition, locomotion, etc., the cetaceans are on the whole the most peculiar and aberrant of mammals. Their place in the sequence of cohorts and orders [of mammalian classification] is open to question and is indeed quite impossible to determine in any purely objective way.”

- from Classification of Mammals

1981: Pakicetus inachus

• Pakistan (48 Mya)• Fluvial sediments

bordering remnants of the Tethys Sea

• Dentition like previously known cetaceans

• Dense auditory bullae with sigmoid process on tympanic & other ear features that are only seen in whales!

1990: Basilosaurus isis

• Egypt (37 Mya)• 243 partial Basilosaurus

skeletons mapped (now 271)• Pelvis bones detached from

sacrum, well-formed joint surfaces on femur

• Most of the bones seen in mammalian hind limbs are present & well-formed

1994: Ambulocetus natans

• Pakistan (47.5 Mya)• Robust limb bones, fused

sacrum, flexible elbow & wrist joints could walk around on land

• Relatively large hind feet, seemingly powerful back & tail powerful swimmer

• More of the skeleton was recovered & reported in 2002

Thewissen et al. 1994

1994: Rodhocetus kasranii

• Pakistan (46.5 Mya)• Articulated skeleton that

included all vertebrae except for end of tail, skull, jaws, pelvis, & shortened femur

• Sacrum lacks fusion between all four vertebrae sign of increasingly flexibility for undulatory swimming

Thewissen et al. 1994

Gingerich et al. 1994

2001: Rodhocetus balochistanensis

• Pakistan (47 Mya)• Vertebrae, virtually complete hands & feet

(composite skeleton made with R. kasranii)• Structure of hands limited terrestrial

locomotion• Expanded feet paddling in water

Thewissen et al. 1994

Gingerich et al. 2001Composite

2001: Artiocetus clavis

• Pakistan (47 Mya)• Virtually complete &

exquisitely-preserved skull• Much of vertebral column &

partial limbs

Thewissen et al. 1994

Rodhocetus Antilocapra Artiocetus

Ankle bones

2001: Pakicetus attockiIchthyolestes pinfoldi

• Pakistan (47 Mya)• Over 350 isolated post-cranial remains have been recovered &

assembled into the following reconstructions

• “Terrestrial…no more amphibious than a tapir” (They have revised their interpretations & now assert that pakicetids were semiaquatic)

Thewissen et al. 1994

Thewissen et al. 2001

Increasing Aquatic AbilityPakicetus

48 mya

Ambulocetus

47.5 mya

Rodhocetus

46.5 mya

Dorudon

40 myaTime

Eocene Whales

Eocene WhalesNumber of

Species

5

2

5

18

13

43 described species!

Summary

• Within the scientific community, while the precise mechanisms for it are debated, evolutionary change is generally assumed

• Evidence for the evolution of life & common descent comes from a variety of sub-disciplines & is constantly accumulating

• In just the past 30 years, a group of mammals whose origin was thought to be “quite impossible to determine in any purely objective way” (Simpson, 1945) has become one of the most convincing & complete examples of macroevolution & evolutionary radiations in the fossil record.

Questions?