The first colonists of land

3-billion-year-old soil bacteria?

The accumulation of small heritable changes within populations over time.

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The first colonists of land

#1  Postby lpetrich » Dec 06, 2017 4:32 pm

Major Clade of Prokaryotes with Ancient Adaptations to Life on Land | Molecular Biology and Evolution | Oxford Academic -- the Terrabacteria (land bacteria).

The Terrabacteria include:
  • Cyanobacteria -- ancestrally freshwater, some of them make spores
  • Gram-positive -- have a thick cell wall that stains positive in the Gram stain
    • Actinobacteria -- some of them make fungus-like strands and spores -- they were formerly called actinomycetes
    • Firmicutes -- some of them make spores -- they include the likes of Clostridium and Bacillus
  • Atypical cell walls
    • Chloroflexi -- green non-sulfur bacteria
    • Deinococcus - Thermus -- D. radiodurans can survive high ionizing-radiation doses with dryness-resistance adaptations
Most other Eubacteria fall into a group that the authors call Hydrobacteria (water bacteria):
  • Proteobacteria -- some of them live in dry conditions
  • Acidobacteria, Bacteroidetes, Chlamydiae, Chlorobi, Fibrobacteres, Planctomycetes, Spirochaetes -- Chlorobi include green sulfur bacteria
There are some early branchers, Aquificae and Thermotogae, but they like very hot environments.

Nevertheless, the earliest branchers in the Terrabacteria phyla are all land and freshwater organisms, and dry environments tend to have a greater fraction of Gram-positive bacteria (in Terrabacteria) than wet ones.

The thick cell walls of Gram-positive bacteria are likely an adaptation to dryness. However, those walls and other dryness adaptations have been helpful for being parasitic on other organisms, causing various diseases. Tuberculosis, for instance, is caused by an actinobacterium, Mycobacterium tuberculosis, and staph infections are caused by a firmicute, Staphylococcus aureus. However, there are plenty of Hydrobacteria infectious agents, like Yersinia pestis, which causes bubonic plague, and Vibrio cholerae, which causes cholera, both in Gamma-proteobacteria.

Human skin bacteria mostly come from these phyla: Actinobacteria (51.8%), Firmicutes (24.4%), Proteobacteria (16.5%), and Bacteroidetes (6.3%). So it's about 3/4 Terrabacteria and 1/4 Hydrobacteria.

Spores are another way to survive dryness, and many Cyanobacteria, Actinobacteria, and Firmicutes make them. Only a few Hydrobacteria make them: Myxococcales in Delta-proteobacteria, and they make spores in slime-mold fashion.

Hyperactive genome-repair systems are another way, and that is used notably by Deinococcus radiodurans, the ray-enduring fearsome berry. It can easily survive 1000 times our lethal dose of ionizing radiation, and it does so with mechanisms that help it survive dryness.

How far back do Terrabacteria go? The authors estimate that the Terrabacteria and Hydrobacteria diverged in the mid-Archean: 3.18 Ga (2.83–3.54 Ga) (Ga = billion years ago). Turning to fossil evidence, there is geochemical evidence of soil bacteria from 2.6 Ga in South Africa, before the Great Oxygenation Event of around 2.45 Ga.

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So the first organisms to colonize land were bacteria, and they did so long before the better-known colonists of land.
lpetrich
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