Layered mounds or columns formed of single-celled organisms and sediments are among the earliest evidence of life on Earth. These 3.5 billion-year-old mounds, called stromatolites, are similar to mounds formed today by microscopic marine life forms

Stromatolites (stro-MAT-o-lites):
These mounds form in sheet-like layers. Sticky mats of thread-like organisms trap grains of sediment, then grow through the "blanket" of sediment particles to form a new sticky layer on top of it.

Mound-building organisms flourished on Earth, building vast stromatolite reefs, until grazing marine animals evolved to prey upon them.

PICTURE CAPTIONS:

• #1: This cross-section shows the intricate structure of a stromatolite from 3.5-billion-year-old fossil deposits in Western Australia.
  Picture courtesy of Ken McNamara, Western Australian Museum
  
• #2: Pillar-shaped stromatolites form today in Shark Bay in Western Australia, a lagoon that's too salty for grazing marine animals.
  Picture courtesy of Ken McNamara, Western Australian Museum
• #3: Branching columnar stromatolites are found in the Coppermine River area of Canada.
  Picture courtesy of KPaul H. Hoffman, Geological Survey of Canada