Darwin, therefore, worried about the rarity of intermediate forms between some major groups of organisms.

Today, many of the gaps in the paleontological record have been filled by the research of paleontologists. Hundreds of thousands of fossil organisms, found in well-dated rock sequences, represent successions of forms through time and manifest many evolutionary transitions. As mentioned earlier, microbial life of the simplest type was already in existence 3.5 billion years ago. The oldest evidence of more complex organisms (that is, eucaryotic cells, which are more complex than bacteria) has been discovered in fossils sealed in rocks approximately 2 billion years old. Multicellular organisms, which are the familiar fungi, plants, and animals, have been found only in younger geological strata. The following list presents the order in which increasingly complex forms of life appeared:

Life Form

Millions of Years Since First Known Appearance (Approximate)

Microbial (procaryotic cells)

3,500

Complex (eucaryotic cells)

2,000

First multicellular animals

670

Shell-bearing animals

540

Vertebrates (simple fishes)

490

Amphibians

350

Reptiles

310

Mammals

200

Nonhuman primates

60

Earliest apes

25

Australopithecine ancestors of humans

5

Modern humans

0.15 (150,000 years)



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