Re: Trees of Life

In message <5emritF38tsgqU1@xxxxxxxxxxxxxxxxxx>, alwaysaskingquestions <alwaysaskingquestions@xxxxxxxxx> writes
There are two ways we know in which life can develop purely from non-living
matter - photosynthesis and chemosynthesis.

Chemosynthesis has wider meanings than are relevant here, but one could characterise photosynthesis and chemosynthesis as the two means by which inorganic carbon can be fixed, that is converted into organic form. One could describe this as conversion of non-living matter into life, but use of the word develop seems to be pushing the bounds of language. If you intend that these are two means by which abiogenesis can occur, then you're confused. We don't know how abiogenesis occurred, but neither term is particularly appropriate.

Why then, do we try to construct a tree of life originating back to a single
point? As organisms using photosynthesis and chemosynthesis are entirely
independent, would it not be more logical to have two separate trees of
life - basically one for plants and one for animals, or do we have real
evidence of a common ancestor? (I'm consciously leaving cyanobacteria out of
this as I don't think it affects my underlying point.)

Leaving out cyanobacteria does affect your point. Photosynthetic organisms form several distinct clades, each characterised by primary, secondary or tertiary symbiosis with cyanobacteria.

The big clade is Archaeplastida (Plantae sensu latissima), which includes rhodophytes (red algae), chlorophytes (green algae) and plants, and glaucophytes, which appear to represent a single acquisition of a photosynthetic cyanobacterium, which evolved into the primary plastid (rhodoplast, chloroplast or cyanelle).

There's a protist, Paulinella chromatophora, which appears to be an independent, and recent, acquisition of symbiotic cyanobacteria.

Another large clade is the chromalveolates, which may (the evidence is not clear cut) represent a single secondary symbiosis between a red alga and another eukaryote. But photosynthesis has been lost in many lineages of this group, and among dinoflagellates it has been independently reacquired several times by symbiosis of a dinoflagellate and another eukaryote (often another chromalveolate). Chromalveolates include brown algae (including kelps), golden algae and other photosynthetic groups.

Chlorarachniophytes are another instance of secondary symbiosis, this time involved a green alga, as is also the case in euglenids.

For an overview of all this, see

* Keeling, Diversity and evolutionary history of plastids and their host, American Journal of Botany 91(10): 1481-1493 (2004)

(available at http://www.amjbot.org)

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Less intimate symbiosis with photosynthetic organisms occurs in a number of other groups, e.g. reef corals, lichens.

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The idea of two separate trees of life for plants and animals is based on a greatly outdated concept of taxonomy. The procrustean plant-animal dichotomy has been long abandoned.

There are three major divisions of life - (eu)bacteria, archaea and eukaryotes. (Some people think that the latter two, collectively neomura, evolved from within bacterial diversity.) There are several photosynthetic groups within bacteria, of which cyanobacteria are only one.

Within eukaryotes, there are perhaps 6 major groups, one of which is the aforementioned Archaeplastida. Another is the Opisthokonta, which includes animals, fungi (including chytrids and haplosporidians as well as the well known yeasts, lichens and mushrooms), choanoflagellates, mesomycetozoans, nuclearids, ministeriids and capsasporids.

And, yes, we do have real evidence for a universal common ancestor.

A secondary area intrigues me. Outside of those basic organisms that use
photosynthesis or chemosynthesis, all other forms of life grow and develop
by consuming other organisms - cow eats grass, man eats cow, etc.

Do we have any knowledge of at what point in evolution this process of
organisms consuming organisms began?


--
Alias Ernest Major

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