Archean Algae Early Age Of Life

ARCHEAN ALGAE EARLY AGE OF LIFE This chapter discusses Earth's early history, its evolving life forms, and the formation of continents. The first 4 billion years, or about nine-tenths of the geologic time (Table 2), are referred to as the Precambrian era, the longest and least understood period in Earth's history. The Precambrian began with only simple creatures living in the sea. It ended with an explosion of new, highly specialized species, which set the stage for more modern life forms to follow (Figure 15) The Precambrian was divided almost equally into millennial Archeans and Proterozoic. The boundaries are somewhat arbitrary and reflect large differences between rock types formed over the two periods. Archean rocks are the product of the rapid formation of the crust. Proterozoic rocks are more representative of relatively stable modern geology. Archean, from 4.6 to 2.5 billion years ago, covers a time when Earth was in great turmoil and experienced extensive volcanism and intense meteorite bombardment. The high internal heat of the newborn planet makes its surface well agitated, destroying the resemblance of the crust. This is why the first few hundred million years do not exist in the geological record. During this interval, the planet experienced restlessness that may have been a major factor in the emergence of life so early in Earth's history. AGE OF ALGAE Life in the Archean consisted mostly of bacteria, unicellular algae, and a group of algae called stromatolites (Figure 16), from the Greek word stroma meaning "rocky carpet." The oldest evidence of life includes microfossils and stromatolites. Microfossils are the remains of ancient microorganisms. Stromatolites are layered structures formed by the accretion of fine sediment grains by tangled colonies of cyanobacteria or primitive bluegreen algae that live in shallow seas. In addition, microbial mats as much as one-third thick grew on the surface of the clay-rich soil, becoming the first life on land about 2.6 billion years ago. Stromatolithic colonies are formed from layers of cells topped by photosynthetic organisms that reproduce using sunlight and supply the lower layers with nutrients. Stromatolites are only indirect evidence of early life. They are not the remains of the microorganisms themselves but only the sedimentary structures on which they build. The earliest stromatolite fossils are in 3.5-billion-year-old sedimentary rocks from the Warrawoona Group Tower Formation in Western Australia's Arctic. The region was once a tidal inlet, overshadowed by high volcanoes erupting ash and lava, which flowed into shallow seas. Thunderclouds hovered above the peak, and lightning shot back and forth. Strong winds whipped up high waves that hit the basaltic cliffs of the coastline. Further inland, hummocks (round piles) of black basalt streams, still steamy from their latest eruption, dominate the landscape. The smell of rotten sulfur eggs permeated. Frequent heavy rains feed the winding tidal flow into a flat expanse of shimmering grey mud before reaching the sea. In others, scattered shallow pools containing highly salty water evaporate periodically, leaving behind various salts. Often, flood waves sweep away mudflats, shifting sediments and replenishing saltwater ponds. Although the Archean covers nearly half of Earth's history, its rocks represent less than 20 percent of the total exposed area on the surface. In addition, all known Precambrian rocks have undergone several episodes of warming and metamorphism. Unlike most ancient rocks in the 3.5 to 3.8 billion-year range around the world, few such as those in the Arctic sequence have a history of low metamorphic temperatures. Therefore, the rocks in this region have remained relatively cold throughout geological history. Rocks subjected to intense heat in Earth's interior have lost all traces of fossil life. Even in slightly metamorphosed rocks, the presence of microfossils, which are preserved cell walls of unicellular microorganisms, is often difficult to prove. Most of these visible fossils are simple spheres with few surface features. They consist of inorganic carbon compounds that are squeezed into spheroids by the growth of mineral granules deposited around them. However, some spheres are connected in pairs or in chains, and others are in groups of four and cannot be created only by inorganic processes. Related to Arctic rocks are cherts, very hard silica rocks that contain microfilaments, which are small, thread-like structures that may be of bacterial origin. Chert is similar to filamentous (thread-like) bacterial microfossils found in the eastern Transvaal, South Africa, and is between 3.2 and 3.3 billion years old. They also exist on the 2-billion-year-old chert of the Gunflint Iron Formation on the north shore of Lake Superior in North America. Most Precambrian chert appears to be chemical sediments deposited from silica-rich water in the deep ocean. The abundance of chert in the Archean probably serves as evidence that most of the crust is heavily submerged. However, cherries in the Arctic region seem to have a shallow water origin. Chert-forming silica leaches from erupting volcanic rocks into shallow seas. Silica-rich water circulates through porous sediments, dissolving native minerals and precipitating silica in place. Microorganisms buried in sediments are thus encased in one of nature's hardest substances, allowing microfossils to survive the rigors of time. Modern seawater lacks silica because organisms such as sponges and diatoms (Figure 17) extract it to build their skeletons. Generally, only silica spicules that make up the sponge skeleton remain as fossils. Diatoms exhibit beautiful glass-like silica lace inside their cell walls. The massive deposits of diatomaceous earth, also called diatomites, composed of diatom cell walls are a tribute to the great success of these organisms after the Precambrian. Arctic stromatolites are clearly layered accumulations of calcium carbonate with a round, cabbage-like appearance. The size and shape of Archean-age microfossils and the shape of stromatolites suggest these microorganisms were oxygenreleasing or sulfur-oxidizing photosynthetic species that depended on sunlight for their growth. Sulfur-metabolizing bacteria may have arisen in the early stages of biological evolution, perhaps about 3.5 billion years ago. Iron metabolism bacteria may be responsible for the large-banded iron formations that formed about 2 billion years ago. Modern stromatolites are in the intertidal zone above the low tide mark. Their length reflects tidal height, largely controlled by the gravitational pull of the Moon.The oldest stromatolite colonies in the Arctic region grow very tall, with some reaching lengths of more than 30 feet. Evidence suggests that at an early age, the Moon orbits closer to Earth, and its strong gravitational pull on this range gives rise to tremendous tides that flood coastal areas far inland. Stromatolites live similarly to ancient times. They consist of concentric coated mounds of calcium carbonate built up by algae or bacteria, which hold sediment grains together by secreting a jelly-like liquid. Older structures at the Australian site are classified as fossil stromatolites or as layered inorganic sedimentary structures. However, microscopic filaments radiating out from a central point resembling filamentous bacteria were also present in the fossil, which suggests the bacteria built stromatolites. Bacteria are the greatest success story in life. They occupy a wider environmental domain and span a wider range than any other group of organisms. They are highly adaptable, indestructible, very diverse, and absolutely necessary for the existence of other life forms. The bacterial way of life has been stable since the dawn of the fossil record. Long after all other species have gone extinct as Earth nears its end, bacteria will remain as they were in the beginning—the only life on Earth.