Precambrian Time Paleobiology

Forward to

Paleozoic Era Paleobiology

Also see:

Geologic Time Clock Precambrian Fossils

Preface - the Sea and Genesis of Life

Any consideration of the geological history of earth as it pertains to the genesis and evolution of life, that is, to paleobiology, must hold the sea as centric. Life began in the sea, and most extant life yet exists in the sea. The sea contains an incomprehensible diversity of life, mostly still undiscovered or described, ranging across all the domains of life. The sea is absolutely brimming with microscopic life, including bacteria that make their living by a constellation of different metabolic processes, and the Archaeans, among which are the extremophiles living in vents at temperatures well above the boiling point of water.

The sea was the mother of all life beginning some 3.8 billion years ago, and remains so today. The land-based animals each carry with them a miniature ocean, pulsing in their cells and circulatory systems. All life, including human, could be viewed as bags sea water containing the same mineral constituency as the ocean together with a dynamic dispersion of molecules that perform the biological processes that constitute life.

In all living cells - proteins answer for both form and function. Proteins are the active elements of cells that aid and control the chemical reactions that make the cell work. They receive signals from outside of the cell. They control the processes by which proteins are made from the instructions in the genes. They also form the scaffolding that gives cells their shape and as well as parts of the linkages that stick cells together into tissues and organs. A protein's shape determines its function, which, in turn, depends on its water-hating (hydrophobic) properties - to work proteins must be immersed in a miniature sea within the cell that does not greatly differ from the sea from whence it came. Life came from the sea, and the sea sustains life on earth, especially the many microbes that recycle the fundamental elements from which proteins are constructed (for example in the nitrogen cycle)

Hadean Time (4500 to 3800 million years ago)

It is believed that the earth formed after the Big Bang some 4 ½ billion years ago (4500 mya), an almost incomprehension amount of time. It was some 4 billion years afterwards, with some exceptions, that animals first left their mark in the fossil record. But it was during this Precambrian period when profound events occurred, leading ultimately to "life" as we know it today. From 4500 to 3800 mya (the Hadean) the earth was indeed a hostile place. During this period the sun formed by gravitational compaction, and eventually reached the temperature and pressure conditions for nuclear fusion. Other particles coalesced under gravity to form continually growing planets. The oldest rocks on earth (3.8 bya) were formed through the cooling of the theretofore, molten Earth. Science is unaware that life existed during the Hadean time, but the prerequisite ingredients for life to emerge were in production. If life did arise during the Hadean, it did so in a truly hellish environment.

Archaean Time (3800 to 2500 million years ago)

The atmosphere that existed during Archean time would be toxic to most extant life on our planet. Also, rocks were just beginning to form at the crust of the earth. It is believed that life on earth made its appearance in the seas during Archaean time. The first life is believed to be the Eubacteria (i.e., bacteria), single-celled prokaryotic organisms with no DNA-containing Nucleus. The most prevalent theory is that the Eubacteria are ancestral to the Archaea, only identified as a distinct domain of life in the 1970's. Domain Archaea include organisms that can exist, and maybe are the only organisms that can exist, in extremely hostile environments, such as thermal vents and hypersaline water. However, not all Archaeans are extremophiles, and, in fact, this domain is extremely diverse, and only recently being studied using genomic and proteomic methods. The earliest primitive bacteria obtained energy through chemosynthesis (ingestion of organic molecules). They produced the oldest fossils that date to about 3500 mya, and are known as bacterial microfossils. Discovered in the 1970s in western Australia, these earliest fossils express what appear to be chemical signs of delicate chains of microbes that appear exactly like living blue-green algae (otherwise known as cyanobacteria). For billions of years, these bacteria formed extensive slimy carpets in shallow coastal waters, and before the end of Achaean-time 2.5 bya had also formed a thin crust on land. Known as stromatolites, these accretionary growth structures produced by the prokaryotes, and also possibly Arachaea and primitive Eukaryotes, became increasingly abundant during the Archaean, a fact of critical importance to the later evolution of life. However, an alternate hypothesis postulates that eukaryotes may have appeared in late Archaean time. Ancient shales of northwest Australia dated with uranium and lead to 2700 mya contain microscopic traces of oil containing sterols. Since eukaryotes are the only organisms on Earth that can make these molecules, these shale's support the theory that amoeba-like eukaryotes may have appeared early in life's history. Stromatolitic structures span the Precambrian and extend to modern time, though they are currently limited to several isolated environments. While science generally can not determine the producing organism or organisms, stromatolite can indeed be beautiful expressions of the most ancient life on earth.

The origins of all modern cells occurred in deep time of the Archaean. Sequence comparisons of proteins thousands of different prokaryotes, together with assumptions of the slow mutation rate of prokaryotes lead to estimates that major classes of primitive microbes (chemotrophs and photosynthetic autotrophs) fused together more than 2.5 billion years ago in a process called endosymbiosis. This endosymbiosis, or symbiotic merging of two cells, enabled the evolution of a highly stable and successful organisms with the capacity to use energy from sunlight through photosynthesis. Perhaps no event is more important to the evolution of life of earth. Further evolution led to increasing diversity of photosynthetic organisms producing oxygen as a byproduct. The resulting oxygenation of Earth's atmosphere profoundly affected the evolution of life, leading to more complex organisms that consumed oxygen, which were the ancestors of all modern oxygen-breathing creatures including humans.

Proterozoic Era (2500 to 544 million years ago)

The Proterozoic, 2.5 bya to 544 million years ago (mya), realized events paramount to the further evolution of life, most notably the steady buildup of oxygen in the atmosphere. Stable continents formed. Bacteria and archaean microbes, some able to tolerate extremely hostile environments, became increasingly abundant. By about 1.8 bya, eukaryotic celled animals appear as fossils, These are the organisms that most people are most familiar with - all animals, plants, fungi, and protists which share fundamental characteristics such as cellular organization, biochemistry, and molecular biology. Cyanobacteria, photosynthetic Eubacteria that produce oxygen as a metabolism byproduct may have appeared of early as 3.5 billion years ago, but became common and widespread in the Proterozoic. The rapid build-up of oxygen in the atmosphere was primarily owing to their photosynthetic activity. Hence, cyanobacteria have been paramount in evolution and ecological change throughout earth's history. They have been attributed at least in part, because of the intense energy density of oxygen-burning aerobic metabolism of Eukaryotes, with the explosion of diversity in the late Precambrian into the Cambrian (the Cambrian Explosion). The other great contribution of the cyanobacteria is in the origin of plants. The chloroplast where plants make food is actually a cyanobacterium living within the plant's cells.

Regardless of whether the Eukaryotes with DNA-containing nucleus evolved in the Arachaen or Proterozic, these ancestors of all plants, animals and fungi are believed to have obtained their energy complex metabolism systems from endosymbiotic bacteria (known as the theory of endosymbiosis). The cellular organelle mitochondria (and associated mitachondrial DNA) of animals, the center of aerobic energy production is believed evolved from aerobic bacteria. Similarly, and in a separate evolutionary event, chloroplasts of eukaryotic plants is believed to have evolved from the autotrophic, photosynthetic cyanobacteria. Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back to 1.6 to 1.7 billion years ago.

Besides endosymbiotic-based metabolism, the other great evolutionary innovation of the Eukaryotes that occured in the Proterozoic was the ability to reproduce sexually, making genetic diversity possible, and as a consequence, greatly enhanced the ability to adapt to and survive environmental changes. Unlike prokaryotic bacteria that are identical clones, sex enabled favorable mutations to persist and amplify in a population's genome, and for multi-celled, soft-bodied marine organisms (metazoans) evolve.

The oldest fossils within Kingdon Animalia are Vendian age 650 to 544 mya, are found at nearly 30 locations around the world, and are most distinctive. The Ediacara Hills of Southern Australia, and the Vendian White Sea Region of Northern Russia are two of the more famous. Typically, the Vendian or Ediacaran fossils are preserved as thin impressions on bedding surfaces of fine to medium-grained sedimentary rocks. Ostensibly, these organisms were very thin, lacked any minerallized hard parts or well developed organs or organ systems, and had a quilt-like outer surface. Some uncertainty exists as to what groups of animals these fossils might represent, and, in fact, if they were ancestral to the multicellular organisms that appeared later in the Cambrian. The so-called Tommotian fauna (biota) appear near the end of the Proterozoic, immediately preceding the Cambrian explosion. These small shelly animals were a prelude to the metazoans with hard exoskeletons that would rapidly diversify in the Cambrian.

To Paleozoic Era Paleobiology