Common Ancestry

All living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure,

and their laws of growth and reproduction… Therefore I should infer… that probably all the organic beings which have

ever lived on this earth have descended from some one primordial form, into which life was first breathed (Darwin,

1859, pg. 484, “On the Origin of Species”)

A. The evidence

• “What could be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones in the same relative positions?”

– (Darwin, 1859, pg. 484, “On the Origin of Species”)

• Evidence for common ancestry comes from “homology”

Homologous bones: 1)Have the same relative position2)Have the same embryological origin

Stretching the same bone in different ways to do different jobs makes sense in the light of evolution…

Human Skull

These skulls are made from the same bones in the same relative positions to each other as in the human skull, but they are re-shaped for different

purposes

The Panda’s “Thumb”

The retina

Not homologous

The evolutionary explanation for embryonic tails…

• http://www.anatomyatlases.org/AnatomicVariants/SkeletalSystem/Images/19.shtml

Coccyx

Fig. 2.4Vestigial structures

Kiwiwings—genes for wings still there and partially functional

Fig. 2.4--Boa legs—genes for leg formation still there and partially functional

Mexican tetra (cave fish)—makes partial eye (not functional)—genes for eye formation still there

http://www.life.umd.edu/labs/jeffery/cavefish.html

Blind cave salamander—makes eyes with retina and lens, but eyelids obscure vision—genes for eye formation still there!

Vestigial hip and leg bones inside whale bodies

Fig. 2-13

Illustration from Man-of-War by Stephen Biesty (Dorling-Kindersley, NY, 1993)

Wounds re-open

GLO gene LOF mutation = “vestigial” gene

Fig. 2.23

We share our genetic “junk” with our closest relatives

Summary

• These shared features make sense if organisms share common ancestry.– e.g., eye development in all fish, whether useful or not– GLO gene sequence present in all mammals, even with LOF

mutations

• If modern organisms were unrelated to each other (as in Lamarckian evolution or as in independent creation), we’d expect “new inventions” for each way of life

B. Common ancestry predicts a nested tree

Fig. 26-3b

Species:Pantherapardus

Genus: Panthera

Family: Felidae

Order: Carnivora