Evolution of Cells and Endosymbiosis

When did the first single-celled organisms appear?

Around 3.5 billion years ago, as shown by fossilized stromatolites.

What are stromatolites?

Layered rocks formed by microbial communities, especially photosynthetic prokaryotes.

What was the significance of early photosynthetic prokaryotes?

They released oxygen, transforming Earth’s atmosphere.

What was the Great Oxygenation Event?

A major rise in atmospheric O₂ about 2.4 BYA, caused by cyanobacteria.

How did oxygen impact early life?

Many anaerobes went extinct, while aerobic respiration evolved.

When did eukaryotic cells first appear?

About 1.8 billion years ago.

What distinguishes eukaryotic cells from prokaryotic cells?

Membrane-bound organelles and a nucleus.

What is the endosymbiotic theory?

The idea that mitochondria and plastids (chloroplasts) originated as free-living prokaryotes engulfed by a host cell.

Who developed the endosymbiotic theory?

Lynn Margulis.

What evidence supports endosymbiosis?

Mitochondria and chloroplasts have double membranes

They contain their own DNA and ribosomes

They replicate independently by binary fission

What type of bacteria are mitochondria thought to have evolved from?

Alpha-proteobacteria.

What type of bacteria are chloroplasts thought to have evolved from?

Cyanobacteria.

What event likely led to the evolution of eukaryotes?

The engulfing of aerobic and photosynthetic prokaryotes by larger host cells.

When did multicellularity evolve?

Around 1.2 billion years ago.

What are the simplest multicellular eukaryotes?

Colonial protists, where cells remained attached after division.

What advantage does multicellularity provide?

Cell specialization and larger size, improving survival and complexity.

What are some early multicellular organisms?

Algae and simple animals found in Precambrian fossils.

What was the Ediacaran biota?

A group of soft-bodied multicellular organisms that lived 600–541 MYA.

Why is the Ediacaran biota significant?

It marks the first evidence of large, complex multicellular life.