tecs article is a really good read. But check out this example of an awfull and crappy writeup:
Life on Earth Arose Just Once
...some scientists have proposed that multiple primordial life forms could have tossed their genetic material into life’s mix, creating a web, rather than a tree of life.
To determine which hypothesis is more likely correct ... The results ... come down overwhelmingly on the side of a single ancestor.
After the first page i was getting more and more critical of the article, untill i read what they actually did:
The theory ofUCAallows for thepossibility ofmultiple independent
origins of life1–6. If life began multiple times, UCA requires a ‘bottleneck’
in evolution inwhich descendants of only one of the independent
origins have survived exclusively until the present (and the rest have
become extinct), or, multiple populations with independent, separate
origins convergently gained the ability to exchange essential genetic
material (in effect, to become one species). All of the models examined
here are compatible with multiple origins in both the above schemes,
and therefore the tests reported here are designed to discriminate
specifically between UCA and multiple ancestry, rather than between
single andmultiple origins of life. Furthermore,UCAdoes not demand
that the last universal common ancestor was a single organism24,25, in
accord with the traditional evolutionary view that common ancestors
of species are groups, not individuals26. Rather, the last universal common
ancestor may have comprised a population of organisms with
different genotypes that lived in different places at different times25.
Hence it is important to keep in mind it is NOT talking about an actual microorganism as the UCA, the UCA is considered a a gene pool in organisms that may not even have lived at the same time, but which was "stirred" by HGT at such a rate it should -across time and distance- be considered one "effective" organism.
