Category Questions

900 MYA

Abundant acritarch fossils, some giant-sized by micro-standards, are thought to be fossil cysts or other life stages of algae. Although we cannot discern exactly "who" they are, they speak to us of life's diversity over millions of years.

Categories are maps, but maps are not the territory. In the game Twenty Questions, the basic categories of animal, vegetable, and mineral underestimate the richness of life-forms. Similarly, basic categories such as "small," "large," "single-celled," and "multi-cellular" can confuse us.

Top: This contemporary alga is an example of one of the largest protoctists. (Photograph by Lois Brynes)

Mid-left: This thin rock section displays fossils 900 million years old. (Photography by Andrew Knoll)

Mid-right and bottom: Photos of ancient filamentous microfossils; the one above and the one to the right are fossil cyanobacteria. (Photography by Gonzalo Vidal, mid-right, and bottom)


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