The History of Life on Earth

Page
2

For example, small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water.

Slide 9

Protobionts May Have Formed Spontaneously

(a) Simple reproduction by

liposomes

(b) Simple metabolism

Phosphate

Maltose

Phosphatase

Maltose

Amylase

Starch

Glucose-phosphate

Glucose-phosphate

20 µm

Slide 10

Self-Replicating RNA and the Dawn of Natural Selection

The first genetic material was probably RNA, not DNA.

RNA molecules called ribozymes have been found to catalyze many different reactions

For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA.

Slide 11

Sedimentary Rocks and Fossils

Sedimentary strata reveal the relative ages of fossils.

The absolute ages of fossils can be determined by radiometric dating.

A “parent” isotope decays to a “daughter” isotope at a constant rate.

Each isotope has a known half-life, the time required for half the parent isotope to decay.

Slide 12

Sedimentary Rock Strata -- Fossils

Present

Dimetrodon

Coccosteus cuspidatus

Fossilized

stromatolite

Stromatolites

Tappania, a

unicellular

eukaryote

Dickinsonia

costata

Hallucigenia

Casts of

ammonites

Rhomaleosaurus victor,

a plesiosaur

100 million years ago

200

175

300

270

400

375

500

525

565

600

3,500 1,500

2.5 cm

4.5 cm

1 cm

Slide 13

Radiometric Dating

Time (half-lives)

Accumulating

“daughter”

isotope

Remaining

“parent”

isotope

Fraction of parent isotope remaining

1

2

3

4

1/2

1/4

1/8

1/16

Slide 14

Radiocarbon dating can be used to date fossils up to 75,000 years old.

For older fossils, some isotopes can be used to date sedimentary rock layers above and below the fossil.

The magnetism of rocks can provide dating information.

Reversals of the magnetic poles leave their record on rocks throughout the world.

Slide 15

The Origin of New Groups of Organisms

Mammals belong to the group of animals called tetrapods.

The evolution of unique mammalian features through gradual modifications can be traced from ancestral synapsids through the present.