by Carl Strang
This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. I include some references, particularly to papers published in the journal Science which commonly is available at public libraries. Contact me if you need sources for other items. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.
Proterozoic Eon (2.5 billion-542 million years ago)
The beginning of this eon formally is marked by the oldest continental rocks that never have been metamorphosed by heat or chemical change. The Proterozoic Eon was officially subdivided in 2004, with the part of it from 640-542 million years ago designated the Ediacarian Period (Science 305:621). The beginning of that period is marked by the end of the “Snowball Earth” glaciation (see below). Other eras and periods have been proposed (4 periods in a Paleoproterozoic Era, 3 in a Mesoproterozoic Era, and 3, including the Ediacarian, in a Neoproterozoic Era).
Life on Earth. Aerobic, eukaryotic life forms appeared in the Proterozoic eon around 2 billion years ago, when oxygen produced by photosynthesis built up enough to require organisms to adapt. Archaea are genetically closer to eukaryotes than are bacteria, and are regarded as the more likely source of the first eukaryotes (Science 311:1283). The oldest eukaryote fossils, at 2.1 billion years, were found in Michigan. This was a significant step, now regarded as resulting from a combination of organisms, with today’s mitochondria and chloroplasts, for instance, the descendents of once separate organisms (for a present-day analog, in which a protozoan consumes an alga and converts it to an indwelling partner, see Science 310:287). Though it has been assumed that this early life was all single-celled, with some colonial forms like stromatolites, in 2010 some marine fossils were reported from Gabon of 2.1 billion years ago that may have been multicellular.
A long time period, from 2 billion to 1 billion years ago, then passed with life stalled at a very simple level. It has been proposed that the cyanobacteria and algae produced oxygen, but it was limited to the atmosphere and upper ocean. It weathered sulfur into the seas, and the resulting sulfides tied up iron and other needed metal nutrients. In particular, iron is needed for nitrogen fixation. The lack of nitrogen limited growth under this theory, and the bulk of the ocean depths lacked oxygen as well, putting that long hold on evolution (Science 297: 1137). An alternative possibility is that an early invasion of land by simple life broke down rock to produce certain clays which buried a significant amount of organic matter. This would insulate it from decomposition by oxygen, allowing the oxygen in the atmosphere to build up and break the long deadlock (Science 311:1446).
The first undisputed eukaryotic, multicellular life forms appeared in the late Proterozoic. These early multicellular organisms, called the Ediacaran biota, were sea creatures, most of them having forms like those of today’s worms, sea pens and jellyfish. Others were unlike anything living today. All known varieties were filter feeders, none preyed on other multicellular species. Study of a series of these fossils from Newfoundland of 560-565 million years ago suggests that the Ediacaran biota were less diverse than had been thought, that previously distinguished species in fact are various body parts, growth stages and environmental variants of the same genetic line (Science 305:1141). A 2010 report of trackways from that area (similar to those produced by sea anemones) suggests that some could move. This appearance of multicellular life followed by only 5 million years an elevation of oceanic oxygen levels that corresponded to an atmospheric content 15% of today’s (Science 314: 1529). In 2010, fossils were reported from Australia that may have been early sponges from 650 million years ago, prior to or during the Snowball Earth glaciation. This is consistent with another 2010 study of sponge genes that found much in common with all animal life and pointed toward a Proterozoic origin of the group.
Local landscape. The most ancient part of North America, the Canadian Shield (extending as far south as central Wisconsin), formed through the collision and fusing of some of the more ancient continental plates, when the Earth had cooled enough in the Proterozoic Eon, 1.95-1.85 billion years ago, that plate tectonics settled down some.
1.83 billion years ago, when the southern edge of North America was in central Wisconsin, a subduction zone dragged a body of continental crust in from the south. The resulting collision enlarged North America, but whether it extended to include northern Illinois is not known. If not, our bit of North America became added 1.8-1.6 billion years ago through further subduction activity accreting new volcanic island arcs in the Middle Proterozoic. With subduction at the local plate boundary a possibility, this was the most likely time for a deep ocean environment here.
Our closest Proterozoic rocks are 4000 feet below us, granite that formed far underground, 1-1.4 billion years ago, as further geological activity resulted from the collision of our continent with others to the south and east (during the formation of an early supercontinent, Rodinia [Science 300:1379]). It is possible that Rodinia’s formation brought a temporary halt to plate tectonic activity, as all subduction zones fused and halted (Science 319:85-88). Without the cooling effect of subducted crust the mantle heated up, with the granite far below us forming as a result. This collision also lifted our area up, so that by the late Proterozoic, that granite was at the surface, exposed to erosion.
The most extensive and severe ice age in the history of the planet is thought to have occurred during this late Proterozoic time, 0.85-0.64 billion years ago. This ice age has been dubbed “Snowball Earth,” and is thought to have occurred because of an incomplete biogeochemical carbon dioxide regulating system. Phytoplankton with calcium carbonate shells had not yet evolved, and their absence at critical times has been connected to too little carbon dioxide in the atmosphere, the inadequate greenhouse gases leading to runaway ice ages (Science 302:859). However, the Snowball Earth period does not appear to have had a major impact on the life that still was restricted to the oceans (Science 300:395). The present consensus is that the planet’s surface was not frozen solid, that there were open places and patches of thin ice that allowed light to reach the ocean’s persisting life (Science 327:1186).
Local life. Granite forms well beneath the Earth’s surface, under continents rather than the sea, but there is the unknown window between life’s origin 3.8 billion years ago and the granite formation 1.5 billion years ago when whatever was replaced by our Precambrian granite could have been a primitive-life-sustaining sea. Our part of the North American continent appears to have been sea floor first. That implies that for an undetermined time period, prokaryotic life was here. Our addition to North America happened at the same time that single-celled eukaryotic life evolved, so there is a slim chance it was here, too, before this area became dry land. There would have been no Ediacaran (late Proterozoic marine multicellular) forms here. There simply is not enough information to allow us to be absolutely certain about whether life was here at all before the current Phanerozoic Eon began.
Fossil cyanobacteria and bacteria, as well as stromatolites are known from as close as northern Michigan, 2.3-2.4 billion years old; stromatolites 1.9 billion years old also have been found in Minnesota. If such organisms were here then, any trace of them was destroyed by volcanism and subsequent erosion. So, in sum: 5-3.8 billion years ago, no life; 3.8-1.5 billion years ago, local life may or may not have existed, if so would have been stromatolites or similar simple marine forms; 1.5-0.5 billion years ago, no local life because it was dry land (unless the clays described above were indeed the result of early microbial activity on land).