The Precambrian encompassed all of early Earth history and rocks from that time preserve evidence of the evolution of life forms on a microbial level. In biology, cell theory states that a cell is the fundamental structural and functional unit of living matter, and that the an organism a multicellular body composed of autonomous cells with its properties being the sum of those of its cells. Multicellular organisms (animals and plants) do not appear in the fossil record until late in Precambrian (Late Proterozoic) time. The Phanerozoic Eon began after the end of the Proterozoic Eon about 540 million years ago, and marks the change when fossil remains of multicellular organisms began to appear in great abundance in the fossil record (discussed below). Geologic Time Highlights of Biological Evolution Formation of the moon in the early Solar System Fig. 2-15. Current thought is that the Moon formed from the debris created by the collision of a small planet-sized object with the ancestral Earth (or Proto Earth) early in the history of the Solar System about 4.5 billion years ago. P R E C A M B R I A N About 4.56 billion years ago Formation of Earth and Moon within the Solar System nebula (Figure 2- 15). (This is discussed in detail in Chapter 1). About 4 billion years ago Evidence of earliest cell-based life of Earth (prokaryotes).
About 3 billion years ago Evidence of photosynthesis and first eukaryotic cells capable of oxygen- based respiration. About 3.0 to 1.8 billion years ago World-wide deposition of banded-iron formations fundamental to the gradual conversion of Earth atmosphere rich in carbon dioxide (CO2) to oxygen (O2) (discussed below). This conversion took nearly a billion years. Once there was enough free oxygen in the atmosphere, this allowed the development of an ozone layer to protect Earth from deadly solar ultraviolet radiation (UV). UV destroys many organic organic compounds. Without an ozone layer, intense solar UV probably would have killed life in the shallow ocean waters. About 1.8 billion years ago Sexual reproduction fully established in eukaryotes. Sexual reproduction increased the rate of mutation in species, leading to increased biodiversity. About 1 billion years ago Earliest evidence of multicellular organisms (metazoans). Early multicellular organism were very primitive but diversified very quickly through geologic time. P H A N E R O Z O I C E O N ⬇ Cambrian Period Beginning about 540 million years ago The beginning of the Cambrian Period started a radiation of species preserved in the fossil record. This is, in-part, because many organisms began to develop the first hard skeletal material as part of defensive and functional body plans. The diversity of species preserved in Cambrian sediments is partly because soft-bodied organisms were not preserved in Precambrian-age sediments.
Significant changes happened in the global physical environment in Cambrian time. Formation of the ozone layer created hospitable habitats and new space for organisms to move up and utilize shallow, warm sea environments that followed a major transgression onto the continents. Organisms were finally
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