In its youth, our planet was molten, red-hot, and battered by abundant meteorites. Earth was so hostile to life that scientists named this period the Hadean Eon (4.6–4 billion years ago), for Hades, the Greek god of the place called Hell.
The earliest known life appears in the geologic record about 3.8 billion years ago in the form of stromatolites, decidedly unsexy blue-green algae, or cyanobacteria. Dating from the Archean Eon (4–2.5 billion years ago) is Banded Iron Formation (BIF)—iron ore containing irregular reddish stripes of stromatolite fossils.
Scientists disagree on how to define stromatolites. Though cellular and photosynthetic, cyanobacteria were neither plant nor animal. They were, perhaps, masses of goo. But the goo hardened, and stromatolites became remarkably resilient fossils.
On Lake Michigan’s Left Coast, excellent examples of Archean BIF can sometimes be found.
Banded Iron Formation, or BIF, from the Precambrian Eon (4600–541 million years ago), by Left Coast Creative (© 2017 Elizabeth G Fagan) for the blog Lake Michigan’s Left Coast