..so what does The LUCA has also been defined as "a hypothetical organism ancestral to all three domains" actually mean? What does the word hypothetical mean in that context?
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All known life forms trace back to a last universal common ancestor (LUCA) that witnessed the onset of Darwinian evolution. One can ask questions about LUCA in various ways, the most common way being to look for traits that are common to all cells, ...
www.ncbi.nlm.nih.gov
The last universal common ancestor between ancient Earth chemistry and the onset of genetics
Abstract
All known life forms trace back to a last universal common ancestor (LUCA) that witnessed the onset of Darwinian evolution. One can ask questions about LUCA in various ways, the most common way being to look for traits that are common to all cells, like ribosomes or the genetic code. With the availability of genomes, we can, however, also ask what genes are ancient by virtue of their phylogeny rather than by virtue of being universal. That approach, undertaken recently, leads to a different view of LUCA than we have had in the past, one that fits well with the harsh geochemical setting of early Earth and resembles the biology of prokaryotes that today inhabit the Earth's crust.
Introduction
The very earliest phases of life on Earth witnessed the origin of life and genetics from the elements. There was a time when there was no life on Earth, and there was a time when there were DNA-inheriting cells. The transitions are hard to imagine. Some dates and constraints on the order of events helps us to better grasp the problem. The Earth is 4.5 billion years (Ga) old [
1]. By about 4.4 Ga, the moon-forming impact turned the Earth into a ball of boiling lava [
1]. Magma oceans with temperatures over 2,000°K forced all water from early accretion into the gas phase and converted all early accreted carbon to atmospheric carbon dioxide (CO2) [
1,
2]. By 4.2 to 4.3 Ga, the Earth had cooled sufficiently enough that there was liquid water [
3]—those first oceans were about twice as deep as today's [
1,
2]. Only later, hydrothermal convection currents started sequestering water to the primordial crust and mantle, which today bind one extra ocean volume [
4,
5]. The first signs of life appear as carbon isotope signatures in rocks 3.95 billion years of age [
6]. Thus, somewhere on the ocean-covered early Earth and in a narrow window of time of only about 200 million years, the first cells came into existence. Because the genetic code [
7] and amino acid chirality [
8] are universal, all modern life forms ultimately trace back to that phase of evolution. That was the time during which the last universal common ancestor (LUCA) of all cells lived.
Conclusions
More clues about LUCA's lifestyle are emerging. Investigations of modern biochemical pathways hone in on the same kinds of reactions as the phylogenetic approach [
103]. Similarly, laboratory experiments also demonstrate the spontaneous synthesis of end products and intermediates of the acetyl–CoA pathway, the mainstay of LUCA’s physiology; new findings show that formate, methanol, acetyl moieties, and even pyruvate arise spontaneously at high yields and at temperatures conducive to life (30°C–100°C) from CO2, native metals, and water [
98,
147]. Those conditions are virtually impossible to underbid in terms of chemical simplicity [
98], yet they bring forth the core of LUCA's carbon and energy metabolism [
78,
96,
97,
101,
103] overnight. Did the origin of genetics hinge upon hydrothermal chemical conditions that gave rise to the first biochemical pathways that in turn gave rise to the first cells? Genes that trace to LUCA [
78], ancient biochemical pathways [
103], and aqueous reactions of CO2 with iron and water [
98,
110] all seem to converge on similar sets of simple, exergonic chemical reactions as those that occur spontaneously at hydrothermal vents [
148]. From the standpoint of genes, physiology, laboratory chemistry, and geochemistry, it is beginning to look like LUCA was rooted in rocks.
You havenotreponded to post #800 and #804
