44

5

Origins of Life and Earth Prehistory

The Origin of Proteins

The random origin hypothesis ³ asserts that proteins originated by stochastic pro-

cesses according to simple rules (i.e., that the earliest proteins were random het-

eropolymer sequences). This implies that their length distribution is a smoothly

decaying function of length (determined by the probability that a stop codon will

occur after a start codon has been encountered, in the case of templated synthesis

without exons). On the other hand, the probability that a sequence can fold into a sta-

ble globular structure is a slowly increasing function of length up to about 200 amino

acids, after which it remains roughly constant. Convolution of these two distributions

results in a length distribution remarkably similar to those of extant proteins.

Geological Eras

Geological eras are summarized in Table 5.2.

Table 5.2 History of the Earth and earthly life

Name

DurationSuperscript normal a^a

Features

New or dominant life

Hadean aeon

4500–4000

Earth formed

None

Archean aeon

4000–2500

Stromatolites

First life

Proterozoic aeon

2500–540

–

Bacteria

Palaeozoic eraSuperscript normal c^c

Cambrian period

540–485

–

Trilobites

Ordovician

485–440

1st MESuperscript normal b^b

–

Silurian

440–420

–

Fish, land (vascular)

plants

Devonian

420–360

Terrestrialization; 2nd

ME

Plants

Carboniferous

360–300

High oxygen

Abundant plants, giant

insects, reptiles

Permian

300–250

Pangaea (the single

supercontinent), hot

and dry; 3rd ME at end

Reptiles; trilobites

disappear

Mesozoic eraSuperscript normal c^c

Triassic period

250–200

Gondwanaland (the

great southern

continent); 4th ME

Dinosaurs

Jurassic

200–145

Warm

Gymnosperms, ferns

Cretaceous

145–65

5th ME at end

Birds, dinosaurs

(continued)

3 See White (1994).