peter jay stein, md, ma
This 54 page synopsis of the evolution
of the Earth, the origin and progression of life, and the
emergence of humans as descendants of the hominin lineage, divides the last 4.57 billion years into progressively overlapping, imagistic, time segments, to outline our place in biological existence.
Emergent Proterozoic Eon: 2500 - 541 mya
Features of the Proterozoic eon - 2500-541 mya:
1) The Earth's daily rotation continues to slow down, from 16 to 18 to a 22 hour cycle, as the moon recedes towards its current distance of 250,000 miles from Earth, and the days lengthen.
(the 3rd, final eon of Precambrian times)
2) Single cell and multicellular eukaryotic (with nuclei and membrane-bound organelles) organisms begin to appear in the fossil record in the rocks. It is now a generally accepted hypothesis that: "...early eukaryotic cells, which were incapable of photosynthesis or aerobic metabolism, enveloped aerobic bacteria...to form endosymbiotic associations...", evolving into...the mitochondria of modern eukaryotes..." (Principles of Biochemistry, 2017, p. 37).
3) In the late Proterozoic era, from 635 mya to 541 mya, just prior to the Cambrian period of the Paleozoic era, Snowball Earth's glaciation ends in the late Cryogenic period, followed by the Ediacaran period, characterized in the fossil record by the appearance of complex, soft-bodied animals.