The Fermi Paradox: Too Rare? II

In my previous post, I had discussed this list up to the origin of life.

  1. Stars I
  2. Planets I
  3. Origin of Life I
  4. Origin of Photosynthesis
  5. Multicellularity
  6. Colonization of Land
  7. Intelligence
  8. Technology
  9. Abstract Science

Here, I’ll be doing up to colonization of land.

Origin of Photosynthesis

To understand it a little better, we must consider the details of energy metabolism that organisms use. It has two main parts: chemiosmotic and electron transfer. Both parts are likely older than the Last Universal Common Ancestor (LUCA), and possibly even the RNA world, and chemiosmotic energy metabolism may have originated from composition gradients in hydrothermal vents, like acidity gradients.

Chemiosmotic energy metabolism works by pumping hydrogen ions out of the cell or into some vesicles in the cell, and letting them return through ATP-synthase complexes. These add phosphates to the phosphate in one of the RNA building blocks. The resulting phosphate-phosphate bonds are then tapped for energy in a variety of metabolic functions, like assembling proteins and strands of DNA and RNA.

Hydrogen ions are typically fed into this process by electron transfer. Electrons are removed from electron sources and transferred into electron sinks, and along the way, the transfer removes hydrogen ions from the cell interior and releases them in the cell exterior or in some interior vesicles.

The two types of photosynthesis I call retinal photosynthesis and chlorophyll photosynthesis.

Retinal photosynthesis is done by only a few organisms, mainly some ones that live in very salty water, ones called halobacteria. Retinal molecules catch photons, change shape, and pump hydrogen ions out of the cell, thus feeding into chemiosmotic energy metabolism. It is thus rather limited.

Chlorophyll photosynthesis is the best-known kind. Chlorophyll molecules catch photons and energize electrons, thus doing a bit of electron-transfer metabolism. The most familiar kind, that of cyanobacteria and chloroplasts, uses two kinds of chlorophyll antenna complexes, with one of them getting electrons from water molecules and releasing oxygen, and sending them to the other one for more energy. There are some bacteria with only one antenna complex, however, and they use different sources of electrons. That suggests some complicated evolution behind cyanobacterium/chloroplast two-antenna photosynthesis, and that may make it difficult to evolve. Chlorophyll itself, however, is built from some common pre-LUCA parts: a porphyrin ring and a terpene.

Oxygen-releasing photosynthesis made possible a major increase in the amount of biomass, because water is much more common than previously-used electron sources. This great bulk of algae and land plants was then able to support large and diversified populations of plant eaters, including animals. This eventually enabled our emergence.

So oxygen-releasing photosynthesis is a potential bottleneck.


One-celled organisms are very short of mental capability, so one needs multicellularity for sentience. But here, one finds an embarrassment of riches. Or so it seems. There are four main kinds of multicellularity:

  • Animallike: one eukaryote
  • Plantlike: several eukaryotes, one prokaryote (cyanobacteria)
  • Funguslike: some eukaryotes, one prokaryote (actinobacteria)
  • Slime-moldlike: several eukaryotes, one prokaryote (myxobacteria)

The first three are all clonal: origin from a single cell or from part of an existing organism. The fourth one is aggregative: lots of one-celled organisms getting together and forming a slug that then makes spores, much like what a fungus does.

What is curious here is that animallike multicellularity evolved only once. Does that mean that its evolution is a bottleneck?

If it is, then there may be planets with big forests and lots of mushrooms but no animals.

Colonization of Land

Communication across interstellar space, and also space travel, require technologies like electricity and metal refining that are easiest on land, so I now consider getting onto land.

The first organisms to do so were most likely various soil bacteria, some 3 billion years ago. They were joined a little over 400 million years ago by a mosslike plant called Cooksonia, and soon after by animals and fungi.

Though animals colonized land several times, and fungi likely also did so, plants did so only once. Thus, there are no land descendants of red algae or kelp, only of some green algae.

I think that there is a big misconception about colonization of land, that it needs ocean tides. It must be conceded that ocean tides provide a halfway house for going onto land, in the form of land that is exposed in low tide and submerged in high tide. But there are some land organisms whose closest relatives are freshwater ones:

  • Land plants (Embryophyta) – Stonewort green algae (Charophyta) – all freshwater
  • Land vertebrates (Tetrapoda) – Lobe-finned fish (Sarcopterygii) – some freshwater, ancestrally freshwater
  • Land snails – less clear, at least some are freshwater
  • Earthworms and leeches – less clear, at least some are freshwater
  • Land arthropods (insects, arachnids, myriapods, isopods separately) – not very clear

Myriapods are centipedes and millipedes, land isopods are woodlice, including pillbugs.

So ocean tides are unnecessary, and that means that a planet does not have to have a big moon to have land organisms.

I’ll leave off here.

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