Precambrian Life
Although the Precambrian contains some seven-eighths of Earth's history, its
fossil record is poor, with the majority of fossils being the
stromatolites that are often heavily metamorphosed or deeply buried. However, preserved cells have been discovered at selective sites, such as the 2.0 billion yar old Gunflint Formation. The earliest life forms were prokaryotes (
eubacteria or archaea) that
evolved in the seas, possibly as early as 3.8 Ba. The first were possibly
chemotrophs existing in an anoxic world and producing H2S or CO2, which were followed by photosynthetic cyanobacteria before the end of the Archaean some 2.5 billion years ago. When the Eukaryotes (single-celled organisms with a nucleus) evolved through
Endosymbiosis is disputed, with claims as early as 3.4 billion years ago, but with less equivocal fossils dating from 1.8 to .8 billion yars ago. With the eukaryotes comes sexual reproduction, enabling genetic diversity and the concomitant ability to adapt to and survive environmental changes. Multi-celled, soft-bodied marine fossil organisms (the
metazoans), the so-called Ediacara fauna, are found in strata dating between 590 to 700 million years ago. The first mineralized fossils appear after the Ediacaran, but before Cambrian begins at around 580 - 590 my; they comprise ambiguous parts, possibly denticles and plates and tubes of unknown affinity and putative calcareous algae. Many of the genes and the proteins they encode are found to be conserved across geologic time from the Precambrian, especially those involved in the most basic cellular functions.
Precambrian Fossils
Archaea CellsThe Most Ancient Life on Earth?
Banded IronArchaean (2.87 bya)
South Pass, Wyoming
Brecciated Banded IronArchaean (2.73 bya)
Jackson County, Wisconsin
Chlorophyll molecules need for photosynthesisArchaean
StromatolitesArchaean (2.58 bya)
Lower Whalen Group, Wyoming
Hadrophycus immanus StromatolitesArchaean (2.58 bya)
Medicine Bow Range, Wyoming
Putative Multicellular MacrofossilsPaleoproterozoic (2.3 bya)
Francevillian Series of GabonStromatolitesPaleoproterozoic (2.3 bya)
Bolivia, South America
Bacteriorhodopsin protein of Halobacteria ArchaeaCollenia StromatolitesPaleoproterozoic (2.3 bya)
Northern Michigan
Gunflint StromatolitesPaleoproterozoic (1.9 bya)
Ontario, Canada
Hox GenesThe Hox genes that control
Metazoan development are highly conserved
Hiemalora stellarisEdiacaran
White Sea, RussiaNemaia simplexPhylum CnidariaEdiacaran
White Sea, RussiaDickinsoniaAnnelid (?)
Ediacaran
White Sea, RussiaParvancorina minchamiArthropod (?)
Ediacaran
White Sea, RussiaThe Estrogen Receptor ProteinMolecular phylogeny dates its origins to well into the Proterozoic
Kimberella quadrataProtomollusc (?)
Ediacaran
White Sea, RussiaG protein-coupled receptorA Primordal Protein Highly Conserved Across Eukaryotes
Cyclomedusa davidiEdiacaran
White Sea, RussiaCloudinaEdiacaran
Omkyk Member, Nama Group, NamibiaThe Archaea comprise a group of single-celled microorganisms that, like bacteria, are prokaryotes that have no cell nucleus or any other organelles within their cells. Consequently, they were once considered to be an unusual group of introduced the three main branches of evolutionary descent as the Archaea, Eukaryota and Bacteria. Classifying Archaea remains difficult, since the vast majority of these organisms have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in environmental samples. classified as a
distinctly separate domain in the three-domain system.
Carl Woese introduced the three main branches of evolutionary descent as the Archaea, Eukaryota and Bacteria. Classifying Archaea remains difficult, since the vast majority of these organisms have never been studied in the laboratory and have only been detected by analysis of their nucleic acids in environmental samples.
Archaeans are an ancient form of life, possibly the most ancient. Putative fossils of archaean cells in
stromatolites have been dated to almost 3.5 billion years ago, and the remains of lipids that may be either
archaean or eukaryotic have been detected in shales dating from 2.7 billion years ago. Since most prokaryotes do not have distinct morphologies, the shapes of fossils cannot be used to identify them as Archaea. Instead, chemical fossils, in the form of the unique lipids found in archaeans are used, and such lipids have now been detected in rocks dating back to the Archaean. The oldest known traces of these isoprene lipids have been found in Greenland, which include sediments formed 3.8 billion years old and are the oldest on Earth; some scientists, however, dispute this claim.The
Theory of Endosymbiosis proposes that Eukaryotic life evolved from the Archaea. That is, the theory explains that organelles such as mitochondria and chloroplasts in eukaryotic cells evolved from certain types of bacteria that prokaryotic cells engulfed through endophagocytosis. These cells and the bacteria trapped inside subsequently evolved a symbiotic relationship. In this endosymbiotic relationship, the bacteria lived within the other prokaryotic cells.
Bacteriorhodopsin is a protein used by
Archaea, most notably Halobacteria (
Domain Archaea, Kingdom Euryarchaeota, Phylum Euryarchaeota, Class Halobacteria). It acts as a proton pump, i.e. it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy. It is the retinal molecule that changes its conformation when absorbing a photon, resulting in a conformational change of the surrounding protein and the proton pumping action.
Halobacteria (also Halomebacteria) are found in water saturated or nearly saturated with salt. They are also called halophiles, though this name is also used for other organisms that live in somewhat less concentrated salt water. They are common in most environments where large amounts of salt, moisture, and organic material are available. Halobacteria can grow aerobically or anaerobically. Parts of the membranes of halobacteria are purplish in color, and large blooms of Halobacteria appear reddish, from the pigment bacteriorhodopsin, related to the retinal pigment rhodopsin. This pigment is used to absorb light, which provides energy to create Adenosine triphosphate (ATP). Halobacteria also possess a second pigment, halorhodopsin, which pumps chloride ions in the cell in response to photons, creating a voltage gradient and assisting in the production of energy from light. The process is unrelated to other forms of photosynthesis involving electron transport however, and halobacteria are incapable of fixing carbon from carbon dioxide.
Archaea evolve extremely slowly, and thus this modern bacteriorhodopsin is likely little changed from deep time on earth.
Collenia Form of Proterozoic Stromatolites from Michigan
Name: Stromatolites - Collenia structure
Geological Time: Precambrian (2.2 to 2.4 Billion Years Old)
Size: 275 mm by 115 mm (maximum)
Fossil Site : Chocolay Group, Chocolay Hills, Northern Michigan
Precambrian stromatolite is the oldest of all fossils, and with much labor (cutting and polishing), it is most beautiful. The banding that commonly appears in stromatolite is a record of the growth patterns of colonies of microorganisms, principally photosynthetic prokaryotes (e.g, cyanobacteria). The colors that are often expressed are the result of the interaction of biological and sedimentary processes, together with subsequent chemistry and mineral exchange.
This particular specimen exhibits a rare structure for stromatolite, the so-called 'Collenia' form genus comprising many small dome structures that has a pattern rather resembling a topological map. Coming from the Chocolay Group in an area known as the Chocolay Hills located southeast of Ishpeming, Michigan, it dates to the Lower Proterozoic, some 2.2 to 2.4 Billion years old. This stromatolitic carbonate has been hand-polished to a mirror finish on one side exhibits intricate banding and a spectrum of reddish-brown colors.
olished Gunflint Stromatolites from Canada
Stromatolites (chertized)
Geological Time: Paleoproterozoic (1.9 Billion Years Old)
Size: 160 mm by 170 mm
Fossil Site: Gunflint Formation, Ontario, Canada
This stromatolite is from the Gunflint Formation, a Paleoproterozoic chemical-clastic sedimentary assemblage outcropping to the immediate northwest of Lake Superior that became famous in 1954 for containing the oldest fossil assemblage known at that time. Older microfossils have since been discovered, but the Gunflint prokaryotes remain one of the most diverse Precambrian fossil communities. In the March 7, 2002 issue of Nature (Volume 406), J.W. Schopf, famous for his role in the discovery of Domain of Life Archaea, reported discovery of cellular remains of prokaryotic cells in the Gunflint stromatolites (combining optically discernible morphology with analyses of chemical composition using laser-Raman spectroscopic imagery). His results remain controversial, however. At about the same time, the age was measured at 1.9 billion years. Notice the veins of fool's gold in the specimen.
Barghoorn, E.S. and Tyler, S.A., 1965: Microorganisms from the Gunflint Chert. Science, vol. 147, p. 563-577.
Schopf, J.W., 1999: Cradle of Life: The Discovery of Earth's Earliest Fossils. Princeton University Press.
Developmental regulation proceeds through the sequential activation of a series of regulatory switches that in turn activate networks of other genes. Many different classes of regulatory genes share a common DNA sequence known as the homeobox, which predates the origin of animals. The best-studied class of homeobox-containing genes are the Hox genes, usually found clustered next to each other along animal chromosomes. A homeobox is a DNA sequence found within genes that are involved in the regulation of development (morphogenesis) of animals, fungi and plants. Genes that have a homeobox are called homeobox genes and form the homeobox gene family. In eukaryotes, a homeobox encodes a protein domain (the homeodomain) which can bind DNA that act as part of transcription factors to switch on cascades of other genes that induce cellular differentiation by initiating the cascades of coregulated genes required to make individual tissues, organs , or body parts. The image above shows a digital rendering of the Antennapedia homeodomain protein from the fruit fly (Drosophila melanogaster) bound to a DNA fragment.
Nemaia Vendian Cnidarian Fossils
Nemaia simplex
Phylum Cnidaria, Class Scyphozoa (?)
Geological Time: Precambrian, Vendian (600 Million Years Old)
Size: Medusoids: 12-20 mm across, Matrix: 105 mm by 104 mm
Fossil Site: White Sea Summer Coast, Archangelsk, Russia
Cnidaria were some of the earliest fossils recognized as animals. This death assemblage of Vendian cnidarians (medusoids) predate the Cambrian Explosion by tens of millions of years. Many of the early attempts at diversity occurred during this late Proterozoic timeframe, and are represented in the fossil record, for example, in the Ediacara Biota of the Flinders Ranges of Australia. Many strange forms were present, some of that seem to not have any synonyms in extant animals. These jellyfish-like examples of Nemaia simplex are relatively attributable to jellyfish descendants alive today. The incredible soft-bodied preservation is believed to be the result of impressions made in a microbial mat contained within the sand, a phenomenon that is exceedingly rare in the fossil record.
Dickinsonia Vendian Fossil
From Russian White Sea Summer Coast
Dickinsonia sp.
Geological Time: Precambrian, Vendian (600 - 550 Million Years Old)
Size: 16 mm X 14 mm
Fossil Site: White Sea Summer Coast, Archangelsk, Russia
This spectacular specimen of early life predates the Cambrian Explosion by tens of millions of years. Many early attempts at diversity were present during this time, both here and in the Ediacara Fauna of the Flinders Ranges of Australia. Many strange forms were present, some of which still do not have any parallels in modern times. This one is known as Dickinsonia, and is found in Vendian (Ediacaran) deposits of both Russia and Australia. While once thought to be an annelid, it may also be interpreted as a Cnidarian similar to the modern-day Fungia. Whatever its true identity, it has been assigned to the Vendozoa, a group of animals that existed prior to the better-known Cambrian Explosion. The incredible soft-bodied preservation is believed to be the result of impressions made in a microbial mat contained within the sand.
Parvancorina minchami Putative Vendian Arthropod
Parvancorina minchami
Phylum: Arthropoda (?)
Geological Time: Vendian (about 555 mya)
Size: 8 mm wide
Fossil Locality: White Sea Coast, Russia
This Parvancorina minchami is a Vendian animal ichnofossil from the White Sea coast in Russia that closely resembles fauna from the lower through middle Cambrian. As such, it supports the appearance of arthropods during or before the Ediacaran. Some impressions suggest there were about 10 biramous legs. There also appears to be affinity for
Burgessia, an arthropod from the
Burgess Shale, as well as apparent similarity to arthropods from the
Kaili biota in China.
Estrogen receptor (ER) bound with estradiol - In the computer rendered graphics above, the small green molecule is Estradiol, the primary 'female' estrogenic hormone, which is part of the Nuclear Receptor Superfamily. The remainder of the image shows the ligand binding domain of the ER protein. The Nuclear Receptor Superfamily protiens include many hormones that have long been known to be highly conserved (i.e., high amino acid sequence homology across vertebrate metazoans) across evolution in vertebrates. Recently, Keay et. al. report (Endocrinology Vol. 147, No. 8 3861-3869) an ER ortholog from an invertebrate, the cephalopod Octopus vulgaris, in which estrogens are produced and play an apparent role. This suggests that the nuclear proteins predate the origin of bilaterally symmetrical animals some .7 to 1.1 billion years ago in the Proterozoic. The estrogen receptor (and its 'brother' the androgen receptor) mediates many endocrine system biological processes both before birth, and throughout life in both males and females. For example, in humans, estradiol is required to make the male brain male during early development; remember, all humans are female in the earliest stages of embryonic development. Molecular phylogenetic studies of protein homology (i.e., amino acid sequence similarity comparisons with assumptions of mutation rate) date the ER and other Superfamily members into the Precambrian. Thus, if molecules could be fossils, the ER above would be a living fossil of truly ancient origins. The main function of the estrogen receptor is as a DNA binding transcription factor that regulates gene expression. However the estrogen receptor has other functions independent of DNA binding. Contrary to what old endocrinology tests assert, the ER is a promiscuous receptor that will bind many small molecule ligands, and many of these are environmental chemicals and pollutants. Such chemicals have been called
Endocrine Disruptors, and adverse effects of such chemicals have been a major scientific concern over the past decade. Estrogen receptors are overexpressed in around 70% of breast cancer cases, which are clinically referred to as ER positive. Estrogen gives cells permission to grow when appropriate. This is essential during puberty, but also necessary in adult life. For instance, estrogen is important in bone growth, and low levels of estrogens can lead to osteoporosis. But in the case of cancer, estrogen can enhance unnatural growth and make the disease worse. The drug tamoxifen is used to treat cancer by blocking the action of estrogen. Tamoxifen is a small drug that mimics the shape of estrogen and binds tightly to the estrogen receptor. When it binds, it changes the shape of a signaling loop on the surface the receptor, preventing such estrogen induced effects as cell division that could worsen cancer.
Also see
Fossil Record for more about the evolutionary origins of the estrogen receptor protein.
Unusual Vendian Protomollusc Kimberella
Kimberella quadrata
Phylum Mollusca (?)
Geological Time: Precambrian, Vendian (600 - 550 Million Years Old)
Size: 18 mm by 7 mm
Fossil Site: White Sea Summer Coast, Archangelsk, Russia
Description: This spectacular specimen of early life predates the Cambrian Explosion by tens of millions of years. Many early attempts at diversity were present during this time, both here and in the Ediacara Fauna of the Flinders Ranges of Australia. Many strange forms were present, some of which still do not have any parallels in modern times. This one is known as Kimberella, thought by some to have had a tough shell much like modern-day molluscs. It has been found associated with Radulichnus which appears to show scrapings much like those made by the radula of a modern snail, as well as traces that may have been made as
Kimberella moved along the seafloor. Whatever its true identity, it has been assigned to the Vendozoa, a group of animals that existed prior to the better-known Cambrian Explosion. The incredible soft-bodied preservation is believed to be the result of impressions made in a microbial mat contained within the sand. Kimberella fossils are also found in the
Ediacara Hills of Australia
Concepts of
natural selection apply to the primary building blocks of life, proteins, in the same manner as to organisms. In fact, the molecular level conservation of proteins across evolutionary time is what leads to conservation of function in the organism. One of the highly conserved proteins is the so-called G protein-coupled receptor shown in the picture. G protein-coupled receptors (GPCRs) are actually a huge protein family (i.e., superfamily) of transmembrane receptors that sense molecules outside Eukaryotic Cells in species ranging from yeast to humans, and activate very basic biological pathways and cellular responses inside the cells. Among the molecules that bind to and activate these proteins are compounds sensitive to light, odors, pheromones, hormones, and neurotransmitters. Because of the many members of the superfamily having diverse functions very basic to life, involving many physiological processes, many drugs are designed to interact with a member of this ancient and critical protein superfamily.
Molecular phylogenetic studies of protein homology (i.e., amino acid sequence similarity comparisons with assumptions of mutation rate) date one primordial form within the family to 600 to 700 million years ago, a time period often conjectured to correspond to the divergence of invertebrate and vertebrate ancestors. The same method estimates that various mammalian G protein-coupled receptors diverged some 90 million years ago, well before the demise of the dinosaurs.
Vendian Fossils Death Assemblage
Cyclomedusa davidi and Hiemalora stellaris
Cyclomedusa davidi & Hiemalora stellaris
Geological Time: Precambrian, Vendian (600 - 550 Million Years Old)
Size: Cyclomedusa 40 mm X 37 mm; Hiemalora 40 mm X 30 mm
Fossil Site: White Sea Summer Coast, Archangelsk, Russia
Description: This spectacular death assemblage predates the Cambrian Explosion by tens of millions of years. Many early attempts at diversity were present during this time, both here and in the Ediacara Fauna ofthe Flinders Ranges of Australia. Many strange forms were present, some of which still do not have any parallels in modern times. These jellyfish-like examples are one that is relatively easy to attribute to descendants alive today. The incredible soft-bodied preservation is believed to bethe result of impressions made in a microbial mat contained within the sand. There is debate about the true nature of both these examples. Cyclomedusa was initially thought to be a pelagic jellyfish, but is now more likely a benthic organism much like a sea anemone.
The one below it was thought to possibly be a member of the Cnidaria by virtue of its tentacle-like structures. Given that Cyclomedsa is not found with preserved tentacles, Hiemalora is now thought to be the anchor of some larger, as yet unknown creature.
