Proterozoic Era

what it used to be and is still evolving over time. You will learn a lot just by studying the different periods, although these are usually broken up further into epochs. Figure 11 shows these in better detail:

PROTEROZOIC ERA

Figure 11.

The Proterozoic era is important in geologic time. There was a lot that happened in this era, extending from 2.5 billion to 541 million years back in time. The tectonic plates rested or floated on magma, but the magma then is different than now. The plates were not as thick as they are now, and the magma was both hotter and more liquid. This

meant that the continents were moving a great deal faster and further than you see them. There was a single continent then, which was Rodinia. Laurentia, the early part of North America, was in the middle of this continent, next to the cratons used to make Antarctica and Australia on the West and next to Africa on the east.

The early life was anaerobic – did not use oxygen – mostly because oxygen was unavailable to them and it would have been deadly anyway. These would have later been crowded out by the archaea organisms, which were photosynthetic.

Eukaryotes soon followed. These organisms were like us in that they had nuclei and clusters of chromosomes instead of just one circular strand of DNA; mostly, however, these were single-celled organisms or at least very simple in the beginning. Many of these organisms used oxygen, while a few were photosynthetic. The end of this eon marked the time when a few multi-celled eukaryotic organisms existed.

Multi-cellularity was important for the progression of life. You could only get so far with single-celled organisms as far as real life-forms on earth were concerned. Prokaryotes like bacteria are not capable of doing much; they are too simple and cannot form multicellular organisms. They also have genetic linkages from mother to daughter that are very different from ours. Eukaryotes can use sexual reproduction, allowing for more diversity in the subsequent generations.

The end of the Proterozoic eon was marked by the coming of what we call Snowball Earth because the earth was almost completely frozen. This was also called the Cryogenic time. It may have come because oxygen replaced the greenhouse gases, cooling the atmosphere dramatically. It also happened because the sun wasn't as strong as it is now, and ice itself is so reflective it would have limited the warmth of the earth, leading to a feedback effect. Once it reached a critical level, the ice would have just made more ice.

The end of snowball earth was called the Ediacaran period after the many fossils showing multicellular organisms coming from hills of the same name in Australia. Others from this time period have been found all over the world —all dating from about 600 million years ago. The fossils found from this time are not like any living thing we now know about. These are all Precambrian fossils.