Revisiting the Drake Equation

[lpetrich]

Meiosis is mentioned only once in the thread, but I think it was key — a low-probability event that may not have happened on most planets with life. Meiosis is a complex process that evolved only once on Earth and was essential before evolution could proceed at adequate speed.

What do you think is so special about meiosis?

Consider photosynthesis, where the initial exciton bounces around haphazardly, "hoping" to make it to the reaction center. The exciton motion is a superposition of various motions, but when the "wave function collapses", the exciton is usually found at the reaction center. Why? It seems like teleology: the reaction center somehow serves as an attractor. Is this correct? If not, why is it that the exciton usually does make it to the reaction center?

Where is the "hoping" from? Some naive popularization? Seems to me that this excitation randomly wanders the antenna complex until it finds the reaction center. That center then absorbs it. No teleology needed.

 

[Swammerdami]

Meiosis is mentioned only once in the thread, but I think it was key — a low-probability event that may not have happened on most planets with life. Meiosis is a complex process that evolved only once on Earth and was essential before evolution could proceed at adequate speed.

What do you think is so special about meiosis?

Meiosis is the mechanism whereby an offspring routinely gets genetic information from TWO parents. Each individual has two alleles for each gene, providing redundancy and reliability, while also facilitating change. Meiosis features chromosomal crossover, which also facilitates evolution. The crossover parameters may be near "Goldilocks" values — too much crossover would leave non-intact, useless genes. It is to facilitate the key process of meiosis that sex and all its trappings evolved!

Life existed on Earth for about 3 billion years before meiosis evolved. This step led to a huge explosion in the rapidity of evolution. The basic process of meiosis is an extension of mitosis—I don't know how many enzymes are needed specific to the sexual reproduction process—but it is key to rapid evolution and, as I said, seems to have been "invented" but a single time during life's history on Earth.

Consider photosynthesis, where the initial exciton bounces around haphazardly, "hoping" to make it to the reaction center. The exciton motion is a superposition of various motions, but when the "wave function collapses", the exciton is usually found at the reaction center. Why? It seems like teleology: the reaction center somehow serves as an attractor. Is this correct? If not, why is it that the exciton usually does make it to the reaction center?

Where is the "hoping" from? Some naive popularization? Seems to me that this excitation randomly wanders the antenna complex until it finds the reaction center. That center then absorbs it. No teleology needed.

I'll give you credit and assume you're not quibbling with the mild anthropomorphism "hoping." (I did put the word in quotes. Perhaps I should have provided also an asterisk and a footnote. )

I'll quote from Life on the Edge by Al-Khalili and McFadden (but adding emphasis). If I oversimplify their view please elucidate it better for us. If they've been refuted please provide a cite:

"If [the energy transfer] heads in the wrong direction ... it will eventually lose its energy rather than delivering it to the reaction center.... It doesn't have very far to its way to its destination before the exciton expires....
The exciton was thought was thought to advance through the chlorophyll forest via the drunkard's strategy. But [this] didn't make much sense as this first event in photosynthesis is known to be extraordinarily efficient ... nearly 100%."​

 

[Underseer]

I think we need to add one more parameter - "willingness to communicate".
Also I think we can be reasonably sure that fl =1. If conditions are suitable, then life is unavoidable.
Intelligence is unavoidable too, once you managed to multicellular stage. What is highly unlikely is technological civilization. There is probably a bunch of planets out there with monkeys running around but no human-like level.

Ah, but why would monkeys evolve to a human-like level on this world but not others?

For the same reason one species of ape on this planet did, but the other apes didn't.

 

[lpetrich]

Meiosis is the mechanism whereby an offspring routinely gets genetic information from TWO parents.

That's not what meiosis is. Meiosis is going from diploid to haploid chromosome quantity.

Normal division (mitosis):
Haploid: (X) -> (XX) -> (X) + (X)
Diploid: (XX) -> (XXXX) -> (XX) + (XX)

The sexual cycle: diploid - haploid - diploid:
Meiosis: (XX) -> (XXXX) -> (XX) + (XX) -> (X) + (X) + (X) + (X)
Cell fusion: (X) + (X) -> (XX)

I'll quote from Life on the Edge by Al-Khalili and McFadden (but adding emphasis). If I oversimplify their view please elucidate it better for us. If they've been refuted please provide a cite:

"If [the energy transfer] heads in the wrong direction ... it will eventually lose its energy rather than delivering it to the reaction center.... It doesn't have very far to its way to its destination before the exciton expires....
The exciton was thought was thought to advance through the chlorophyll forest via the drunkard's strategy. But [this] didn't make much sense as this first event in photosynthesis is known to be extraordinarily efficient ... nearly 100%."​

That argument is pure handwaving. It has nothing on antenna-complex exciton lifetimes, whether time to de-excite or time to reach the reaction center.  Light-harvesting complexes of green plants states that these complexes have 250 - 400 pigment molecules, and an exciton presumably travels from one of them to one of its neighbors, then repeats that process for as long as it continues to exist.

A simple random-walk analysis suggests N² steps before exit. That's rather large-looking but then again, I don't have numbers for its lifetimes.

 

[Swammerdami]

Meiosis is the mechanism whereby an offspring routinely gets genetic information from TWO parents.

That's not what meiosis is. Meiosis is going from diploid to haploid chromosome quantity.

Normal division (mitosis):
Haploid: (X) -> (XX) -> (X) + (X)
Diploid: (XX) -> (XXXX) -> (XX) + (XX)

The sexual cycle: diploid - haploid - diploid:
Meiosis: (XX) -> (XXXX) -> (XX) + (XX) -> (X) + (X) + (X) + (X)
Cell fusion: (X) + (X) -> (XX)

Talk about missing the forest for a tree!
Whether you think of Meiosis or Cell fusion or both as the key "invention(s)"(*) that led to sexual reproduction, this was a prerequisite for fast evolution, and therefore advanced life.

(* - Yes, "invention" is another anthropomorphism. I've left it as a nit for you in case you find no other nit to pick. )

 

[lpetrich]

Then why did you say "meiosis"? It's only part of the sexual cell cycle, and I fully concede that cycle's value in creating new combinations of variations.