Hadean

Based on radio-dating of meteorites, the solar system is about 4600 Ma (4600 million years), or 4.6 billion years old (Ga). Condensation of the solar system from a large, hot accretion disk occurred about 10 Ga after the Big Bang.

When early Earth was cooling from its molten state, conditions would have been like hell on Earth – hence the term "hadean" for this earliest segment, or eon, of the Precambrian. Meteors rained down on the hot Earth, ultraviolet radiation was unchecked by an ozone belt, and volcanos belched fumes into a reducing atmosphere, which sparked with lightning.

The term "hadean" was coined to designate the time before the earliest known rocks. However, rocks have been found that are older than the time-frame of the Hadean.

Life, of course, had not yet arisen by biopoiesis, but it probably had an earlier start than thought previously.

The Hadean Eon extends from Earth's formation to 3.8 billion years ago (Ga) and is succeeded by the Archean Eon (not to be confused with prokaryotic Archaea), which lasts until 2.5 Ga. The Proterozoic Eon next extends from 2.5 billion years ago to 540 million years ago, and is succeeded by the Phanerozoic ('visible life') Eon.

αΩ Beginning αΩ Geological Time αΩ Ediacara ▫ site map

tags [Evolution] [Hadean Eon]

| 0 Guide-Glossary

abiogenesis

In current biological terminology "abiogenesis" refers to the emergence of life as self-replicating assemblages of organic chemicals able to control chemical energy from non-life assemblages of chemicals. This is the hypothesis that primordial life originated within the first billion years of Earth's history as a result of chemical reactions that generated larger and larger organic polymers, which ultimately attained control of bioenergetics and the property of chemical self-replication. This modern conceptualization is reasonable in view of what is now known of the biochemical basis of all life.

image : joke based on the magic thinking of creationism

poke the can to continue, or proceed directly to fossils

αΩ Beginning αΩ Geological Time ▫ site map

tags [Evolution] [abiogenesis] [biopoiesis]
[Origin of Life]

| 0 Guide-Glossary

LUCA

The earliest life forms (gray '?') were probably in existence at least 4 billion years ago (4 Ga), and, as depicted, would have evolved along different branches.


The last universal common ancestor (LUCA), or universal cenancestor, depicted with a yellow '?' is the prokaryotic organism hypothesized as being at the ancestral root of all living organisms. Not the earliest or simplest living organism, and not necessarily the sole example of its type, this organism possessed the genetic material that diverged (about 3.5 Ga) into all current living organisms. A number of terms are employed to refer to the universal cenancestor – last universal ancestor (LUA), last common ancestor (LCA), or last universal common ancestor (LUCA).

αΩ Beginning αΩ Geological Time ▫ site map

tags [Evolution] [universal cenancestor] [LUA] [LCA] [last universal common ancestor]

| 0 Guide-Glossary

phylogenetic tree

LUCA ultimately gave rise to the prokaryotic Bacteria (Eubacteria) and the Archaea.

Phylogenetic separation into evolutionary relationships (clades), based on comparison of genomes is likely to supplant phenotypical (phenetic) taxonomies of the prokaryotes. Phenetic systems group organisms based on mutual similarity of phenotypic (physical and chemical) characteristics. Phenetic groupings may or may not correlate with evolutionary relationships. The image above left shows a phylogenetic tree of living things, based on 16s rRNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes.

Trees constructed with other genes are generally similar to the 16s rRNA tree, although they may place some early-branching groups very differently due to analysis problems arising from long branch attraction. The exact relationships of the three domains are still being debated, as is the position of the root of the tree. It has also been suggested that due to horizontal (lateral) gene transfer, a tree may not be the best representation of the genetic relationships of all organisms.

Long branch attraction (LBA) is a problem in phylogenetic analyses, particularly in those analyses employing the non-parametric statistical method termed maximum parsimony. In LBA, rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. This problem in analysis arises when the DNA of two (or more) lineages evolves rapidly. Because there are only four possible nucleotides, high rates of DNA substitution create the probability that two separate lineages will convergently evolve the same nucleotide at the same locus. In such cases, parsimony erroneously interprets this similarity as a synapomorphy, that is, as having evolved once in the common ancestor of the two lineages. The problem of LBA can be minimized by applying statistical methods that incorporate differential rates of substitution among lineages, such as maximum likelihood, or by breaking up long branches by adding taxa that are related to those with the long branches.

A synapomorphy is a derived character-state that is shared by two or more terminal groups – taxa included in cladistic analyses as being units that cannot be further divided. The pertinent character-states in synapomorphy are inherited from their most recent common ancestor.

αΩ Beginning αΩ Geological Time ▫ site map

tags [Evolution] [phylogenetic tree] [evolutionary relationship] [clade] [phenetic] [taxonomy] [domains of life]

| 0 Guide-Glossary

evolution of oxygenic photosynthesis

About 3.5 billion years ago (Ga), LUCA gave rise to the two prokaryotic domains – Eubacteria (more commonly called bacteria) and Archaea (formerly archaeobacteria, commonly called 'extremophiles' because of their persistence in extreme environments).

Oxygenic photosynthesis first arose in the Cyanobacteria at least 2.8 to 2.7 Ga, and gradually converted the Earth's atmosphere from its primordial reducing state. Oxygen is toxic to organisms that lack the metabolic machinery to rapidly utilize oxygen, so coexistent organisms faced the challenge of coping with rising atmospheric levels of toxic oxygen. Serial Endosymbiosis Theory (SET) is a widely accepted, evidence-based theory of cellular evolution that explains the cellular evoluton of eukaryotes through the endosymbiotic union of engulfed bacteria with a precursor eukaryotic cell. The archaeobacteria are more similiar than are eubacteria to eukaryotic cells, so it is believed that eukaryotes sprang from an archaeobacterial eukaryotic-precursor cell.

Symbiotic unions with bacteria capable of oxidative metabolism would have ensured cellular survival in environments with increased oxygen tension (pO2). Serial endosymbiotic transfers ultimately resulted in mitochondrial organelles, while another union with Cyanobacterial cells ultimately resulted in photosynthetic plastids (chloroplasts).

Cyanobacteria are found in the earliest known microfossils, and built large stromatolite reefs, dominating life for more than 2 billion years (~3.5 Ga to ~1 Ga). Modern stromatolites exist in only a few stressed environments on the planet, though Cyanobacteria remain relatively plentiful among the prokaryotes. ∩ Cyanobacteria and stromatolites ∩ Stromatolite structure
∩ Ancient stromatolite reefs ∩ Imaging fossil cyanobacteria ∩ Stromatolite fossils

Stromatolites at Virtual Fossil Museum

αΩ Beginning αΩ Geological Time ▫ site map

tags [Evolution] [evolution of photosynthesis] [stromatolite] [Cyanobacteria] [Serial Endosymbiosis Theory] [SET] [microfossils]

| 0 Guide-Glossary

geological time

Geologists have based geological timelines on significant events in Earth history, as a result the named geological intervals are not of equal length.

Eons are the longest intervals (hundreds of millions of years) and are subdivided into eras, which are further subdivided into periods, and still further subdivided into epochs.
mnemonics

By convention: Precambrian Eons are:
The Hadean Eon, which extends from Earth's formation to 3.8 billion years ago (Ga) is succeeded by the Archean Eon (not to be confused with prokaryotic Archaea), which lasts until 2.5 Ga. The Proterozoic Eon next extends from 2.5 billion years ago to 543 million years ago.

From 543 million years ago to the present geological time is classified as the Phanerozoic (visible life) Eon, and the eon opens with the Cambrian Period.

Geological Society of America time scale - pdf
Geologic Time Scale from International Commission on Stratigraphy
Geologic Time Scale - University of California Berkeley
Geologic Time Scale - US Geological Survey
Paleontology and the Geologic Time Scale - US Geological Survey

PaleoMaps Precambrian Cambrian Ordovician Silurian Devonian Early Carboniferous Late Carboniferous Permian Triassic Jurassic Late Jurassic Cretaceous K/T extinction Eocene Miocene Last Ice Age Modern World Future World Future +100 Future +250

αΩ Beginning αΩ Hadean αΩ abiogenesis αΩ LUCA αΩ phylogenetic tree αΩ evolution of oxygenic photosynthesis αΩ geological time αΩ timeline of life αΩ fossilization αΩ Ediacara Biota ▫ site map

tags [Evolution] [geological timeline]

| 0 Guide-Glossary

timeline of life

4.6 billion years ago the sun and planets condensed from a large, hot accretion disk.

Earth's earliest atmosphere of H2 and He was lost to space, and was replaced by a reductive atmosphere with a composition probably similar to outgassing of modern volcanoes – H2O, CO2, SO2, S2, Cl2, N2, NH3, and CH4.

Oxygen levels began to rise after the evolution of oxygenic photosynthesis by the Cyanobacteria, which evolved at least at least 3450 million years ago (3.45 Ga) and formed the earliest microfossils as stromatolite reefs.

Computer simulations that examine the effect of oxygen on metabolic networks demonstrate that many of the complex biochemical networks employed by advanced organisms could not have evolved without oxygen. The largest and most complex of these networks require the presence of molecular oxygen. Comparatively smaller, simpler networks encompass anoxic, or oxygen-free, pathways common to all life, from single-celled bacteria to the largest mammals. [n]

As Cyanobacteria poured oxygen into the early atmosphere, horizontal gene transfer probably enabled some prokaryotes to acquire oxygen-metabolizing machinery. There is considerable evidence that the earliest eukaryotes evolved through serial endosymbiosis. Chloroplasts resulted from endosymbiotic transfers of Cyanobacteria, and mitochondria originated from endosymbiotic transfers of alpha-proteobacteria (purple bacteria). Mitochondria are the site of oxidative phosporylation in eukaryotes.

αΩ Beginning αΩ Hadean αΩ abiogenesis αΩ LUCA αΩ phylogenetic tree αΩ evolution of oxygenic photosynthesis αΩ geological time αΩ timeline of life αΩ fossilization αΩ Ediacara Biota ▫ site map

tags [Evolution] [primordial atmosphere] [evolution of oxygenic atmosphere]

| 0 Guide-Glossary

fossilization

The processes of fossilization (taphonomy) preserve evidence of organisms as mineralized or imprinted traces within sediments. Fossils may be altered remains of the actual organism or imprints such as body impressions, trails and footprints, or holes made by burrowing creatures.

(Left - fossilization by various mechanisms with trace fossils at center.)

▼ ammonites : biostratigraphy : creationist fallacies : discovery of fossils : fossilization potential : graptolites : index fossils : lagerstätte, lagerstätten : taphonomy : trilobites ▼

Fossilization potential (FP) is the likelihood that an organism will leave some trace of its existence, and this is increased by:
● hard body-parts (exoskeletons, endoskeletons)
● marine rather than land habitat (longer history of marine life, greater likelihood of burial in sediments)
● possibility of rapid burial by sediment (slides, volcanic ash)
● burial of soft-bodied organisms in fine-grained sediment (imprints)
● anoxic environment at burial (swamps, tar sands, resin, rapid burial under sediment)

● 'mummification' by desiccation, ice
● perfusion of burial site by mineralizing solutions (petrification)

The chances that a particular fossil remain will be discovered by paleontologists and earth scientists depends upon:
● geologic stability of biostrata

● protection of soft body parts from scavengers and decomposition
● erosion of overlying strata
● freedom from permanently overlying snow or ice
● geographic location and accessibility (physical, political, educational)

Index fossils are particularly useful in biostratigraphic correlation of the age of rock formations [diagram, shale fossil index]:
● rapid evolution constricts the timeframe (range) during which a particular species lived, enabling more exact identification of the relative age of strata
● abundant and widespread occurrence relatively unaffected by environment increases the likelihood that the species will have fossilized across different rock strata formed contemporaneously
● easy identifiability aids cross-correlation

Index fossils, or guide fossils include:
● graptolites (Paleozoic)
● trilobites (extinct arthropods from Cambrian to Permian extinction)
● ammonites (cephalopods from Late Silurian or Early Devonian to KT-boundary)
● some corals
● brachipods (Paleozoic)
● some echinoids (e.g., Micraster in Cretaceous)
● conodonts

While a lagerstätte (pl. lagerstätten) is a fossil bed that displays exceptional quality of preservation, the vast majority of organisms that have lived have left no fossilized traces or their fossilized remains will never be uncovered or discovered. The fact that relatively few organisms leave any trace has been twisted by creationists in fallacious arguments against the fact of biological evolution.

◘ biostratigraphy ◘ Timeline Earth Life ◘ Hadean ◘ Archaean ◘ Proterozoic ◘ Phanerozoic

Table  Lagerstätten & Biota

Burgess Shale : Canowindra : Chengjiang Lagerstatte : Doushantuo : Florissant formation : Gilboa : Gogo Formation : Grübe Messel : Hunsrück Shale : Holzmaden : Karatau : Lake Turkana : Las Hoyas : La Voulte-sur-Rhône : Liaoning : Ludford Bonebed : Mazon Creek : Monte Bolca : Princeton Chert : Riversleigh Fossil Sites : Rhynie Chert : Santana Formation : Sirius Passet Formation : Solnhofen : Soom Shale : Tlayúa

: WHO'S ON FIRST? A RELATIVE DATING ACTIVITY :

| 0 Guide-Glossary