First Molecules and Cells

Objectives

• Describe the fossil record
• Describe the role of molecular clocks
• Summarize the geological time scale
• Describe the RNA world hypothesis
• Describe the oxygen catastrophe

Fossils are

• preserved remains OR
• traces of organisms

The conditions required for fossil formation are very rare.

The fossil record is the record of life that unfolded over 4 billion years and has been pieced back together.

Fossils must be dated to be useful

• Relative dating
• Absolute dating

A molecular clock uses DNA sequences or proteins to estimate the relatedness between species.

Molecular clocks estimate the time when when related species diverged from a common ancestor.

Species that have accumulated a greater number of differences are assumed to have diverged from their common ancestor in the more distant past.

The geological time scale organizes Earth's history and the evolution of life on the basis of important events, not just time.

First Organic Molecules

Early Atmosphere

• Ammonia
• Methane
• Water vapor
• Carbon dioxide

Scientists hypothesize that lighting struck the atmosphere, creating a "soup" of organic molecules.

In 1953 Miller and Urey tested the hypothesis.

Within a week, several organic molecules had formed.

Which came first?

• Chicken or the egg?
• DNA or proteins?

Some scientists believe RNA was the first organic molecule; this is the RNA world hypothesis.

Untitled Slide

• RNA can encode genetic instruction
• Some RNA can carry out chemical reactions

First Cells

First Cells

• Likely primitive prokaryote-like
• Possible that lipid membrane grew around RNA

Likely one cell (or group of cells) gave rise to life on Earth. Referred to as Last Universal Common Ancestor (LUCA).

LUCA

• 3.5 billion years ago
• One of the earliest prokaryotic cells
• Lacked nucleus and organelles

Earliest Cells

• Probably heterotrophs
• 3 billions years ago autotrophs evolved

Once photosynthetic abilities evolved, oxygen began to accumulate, creating the oxygen catastrophe.

Oxygen was toxic to most early cells because they had evolved in its absence. Most died out.

Cells that survived developed a way to take advantage of oxygen: cellular respiration.