EARLY ATMOSPHERE, EARLIEST ANIMAL LIFE, MODES OF FOSSILIZATION
If the earliest atmosphere was of volcanic origin, it must not have contained oxygen gas (O
), which is
not emitted from volcanoes.
Thus, the earliest organisms must have been anaerobic (able to live in the
absence of free oxygen).
Photosynthesizers emplaced oxygen, a by-product of photosynthesis, into the
Only after a substantial buildup of O2 had occurred was it possible for animals, which are
aerobic organisms, to exist.
At first, the oxygen promptly combined with dissolved iron ions, causing hematite (Fe
) or magnetite
) to precipitate.
Ferrous iron (Fe
) dissolves in water, whereas compounds that contain ferric
, including minerals consisting of iron oxide (hematite and magnetite), are generally highly
This iron of sedimentary origin forms the world's greatest iron ore deposits.
Only much later
(perhaps at the Precambrian-Cambrian transition?) did oxygen begin to accumulate significantly in the
Multiple-celled organisms (all of them eukaryotes) appeared in the fossil record nearly two billion years
after the early prokaryotes.
There is a fossil record of animals, known as the Ediacara (E-dee-ACK-ara)
fauna that lived during latest Precambrian time.
Many of these soft-bodied creatures had a very large
surface area compared to body volume, and a number of them had body plans that are totally unknown
For example, the body of one animal consisted of inflatable interconnected compartments like an
Lacking jaws and teeth, these animals were passive feeders.
They could not be predators,
but rather, they waited for food to be wafted to them.
During Middle Cambrian time, fossils of numerous animals, both with and without hard parts, were
elegantly preserved as the Burgess Shale fauna, in western Canada.
Fossilization occurred by occasional
underwater mudslide events that instantaneously smothered the organisms.
They included a balanced
ecological community with prey, predators, scavengers, filter feeders, mud grovelers, swimmers, etc.
The Burgess Shale fauna also included organisms with no modern analogs -- for example, a critter with
We postulate that when animals first appeared there was an evolutionary explosion of "experimental"
forms, some of which survived to this day (echinoderms, mollusks, arthropods, etc.), and some of which
We do not know what adaptive advantages the surviving forms must have had relative
to organisms that became extinct.
PRINCIPLES OF STRATIGRAPHY
Before geology became an established science, people had incorrect notions about
study of strata.
They conceived of concentric layers wrapping around the earth like onionskins.
Another incorrect notion was that the oldest rocks "look" older (more lithified, etc.) and that young