The first colonists of land: early bacteria

[lpetrich]

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.


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

 

[BH]

Granted bacteria settled on land before anyone else. But did multicellular life colonize freshwater rivers and streams before the land?

 

[lpetrich]

Granted bacteria settled on land before anyone else. But did multicellular life colonize freshwater rivers and streams before the land?

Yes, very likely.

The closest relatives of land plants are  Charophyta algae, which are all freshwater.

For animals, it's been much more difficult to find anything on freshwater vs. marine ancestry of land ones.

• Nematodes
• Annelids: clitellates (earthworms, leeches, etc.)
• Mollusks: gastropods: pulmonates (land snails)
• Arthropods: (several)
• Vertebrates: lobe-finned fish -> tetrapods

But some land animals have many freshwater relatives, animals like land snails and clitellates.

 

[BH]

Yeah, like lobsters to crawfish to scorpions.

 

[lpetrich]

Yeah, like lobsters to crawfish to scorpions.

Lobsters to crawfish, yes. To scorpions, no, because their mouthparts are too different. So their pincers are a case of convergent evolution. They have the same structure because they are derived from outermost limb segments.

On land crabs,  Terrestrial crab,

There is no clear distinction between "terrestrial", "semi-terrestrial", and "aquatic" crabs.[2] Rather, there is a continuum of terrestriality displayed among the true crabs, although even the most land-adapted crabs must still return to water to release their eggs. ...

Terrestrial crabs have often evolved from freshwater crabs, since the physiological changes needed for living in fresh water are pre-adaptations for terrestrial living.[5] On some oceanic islands, terrestrial crabs occupy the top of the energy pyramid.[2]

That article linked to this article on African freshwater crabs.

Terrestrial activity

Many species actively forage on land, and several species have become semi-terrestrial. Ability to reabsorb salt from the urine and to restrict water loss, both of which are physiological adaptations to fresh water, have preadapted these crabs to terrestrial life (Morris & van Aardt 1998).

Furthermore, in addition to gills most species possess a pseudo-lung, a modified gill cavity allowing them to breathe in the air (Adamczewska et al. 1997), and in extreme cases the pseudo-lung has developed to the extent that it allows some species to be almost exclusively terrestrial (e.g. Cumberlidge 1986, 1991). The main restriction to complete terrestrialisation in freshwater crabs is their inability to excrete ammonia into the air. They need to return to water periodically to perform this function, but when they do so the rate of excretion can be up to 70 times faster than normal, requiring only a brief period of immersion (Morris & van Aardt 1998).

Note that living in freshwater means going part of the way to living on land. Having pseudo-lungs is a way of surviving in swamps and marshes, where the water can get short on oxygen.

 

[lpetrich]

Some other crustaceans now live on land, notably the pillbugs. These are in a taxon called Isopoda, and many of their fellow isopods live in freshwater or the oceans ( Isopoda, Isopoda - Tree of Life project). The land isopods are in taxon Oniscidea ( Woodlouse), and it is not very clear whether their closest aquatic relatives are freshwater or marine.

Insects are all land animals, and their closest crustacean relatives are not very clear. Likely an obscure group of them called the remipedes.

Myriapods (centipedes, millipedes, ...) are likewise all land animals, and their closest aquatic relatives are also obscure. Likely the crustaceans, however.

Chelicerates include spiders, scorpions, ticks, mites, and other arachnids, and also horseshoe crabs and sea spiders (pycnogonids) and the now-extinct sea scorpions (eurypterids). Of these, arachnids are mostly terrestrial, while the others are mostly marine. Not much of a hint there either.

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About mollusks, the only land ones are pulmonate gastropods, those with lungs. The terrestrial ones, land snails and land slugs, are mixed in with freshwater and marine ones: Phylogenetic relationships and evolution of pulmonate gastropods (Mollusca): New insights from increased taxon sampling - Phylogenetic_relationships_and_evolution20151028-10720-197wtpb.pdf

About annelids, I've found this paper: Terrestrial polychaetes — models for the evolution of the Clitellata (Annelida)? | SpringerLink There are some land ones, and they have several clitellate-like features. Clitellates themselves are mostly freshwater or terrestrial, with a few marine ones. Clitellates include earthworms and leeches, and they are distinguished by their clitellum or collar for secreting egg cases. So these worms are likely freshwater-to-land.

 

[lpetrich]

There is something weird about vertebrates in this context.  Osmoregulation has some background. Consider a semipermeable membrane placed in water with various materials dissolved in it. Water molecules can pass through those materials, but those dissolved materials' molecules cannot. Dissolved materials = solute. If one side has more solute than the other, then the side with more solute has less water exiting it for the other side, and the side with less solute has more water doing so. The more-solute side thus has less osmotic pressure than the less-solute side.

So water tends to leak into the more-solute side from the less-solute side, stopping when both sides have the same solute concentration.

Let's see what happens.

Marine invertebrates have roughly the same salt content as the ocean around them, and among vertebrates, hagfish are like that also. But lampreys and jawed vertebrates are different. They have only 1/3 the salt content of the surrounding ocean. This means that they risk the Ancient Mariner syndrome, dehydrating in a watery environment. Bony fish cope with that by drinking seawater and excreting very salty urine, while sharks and rays cope with that by accumulating enough urea in their blood to give them the same osmotic pressure as the surrounding ocean.

So why did these fish saddle themselves with a low salt concentration? There is a simple solution: their ancestors lived in freshwater, and they swam back into the ocean. In freshwater, there is the opposite problem, drowning in it. Reducing one's solute content makes it easier to manage the excess water. So some early fish swam out of the ocean into some rivers and lakes and became adapted to the low salt content by this means -- an adaptation that their descendants have been stuck with ever since.

 

[lpetrich]

I've found
Information on Fish Evolution

The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 395 million years ago. The early forms were freshwater fishes, for no fossil remains of modern bony fishes have been found in marine deposits older than Triassic time, about 230 million years ago. The Osteichthyes may have arisen from the acanthodians. A subclass of the Osteichthyes, the ray-finned fishes (subclass Actinopterygii), became and have remained the dominant group of fishes throughout the world.

About ray-finned fish, | Proceedings of the Royal Society of London B: Biological Sciences infers that from about 300 to 200 million years ago, they were all freshwater, and thus that the ancestral ray-finned fish was a freshwater one. Fossil-based comparative analyses reveal ancient marine ancestry erased by extinction in ray-finned fishes - Betancur-R - 2015 - Ecology Letters - Wiley Online Library claims that the ancestors of some of the early-branching ones had been marine, but that they returned to freshwater.

It was the lobe-finned fish, like lungfish and coelacanths, that gave rise to land vertebrates. Lungfish now have a pan-Gondwana distribution, with 1 South American species, 4 African ones, and 1 Australian one. They go back to the Devonian Period, with the earliest ones being over 400 million years old. So they were likely freshwater over all that time.

As to the closest fish to the earliest tetrapods, they were usually freshwater. This includes some of the earlier ones, like Eusthenopteron, from 385 million years ago (Late Devonian). Though it was aquatic, some close relatives started crawling out of water some 10 - 20 million years later.