scigirl

yguy asked this question in another thread:
Any takers?

scigirl

Jesus Tap-Dancin' Christ

How did the first cell come to be?

No one knows--no one will ever know for certain.

What does probability have to do with it? Everything. Probability rules the world, my friend. Every reaction of every sort is governed by a percent chance of occuring or not occurring at a small enough scale. Everything.
So what does probability actualyl play? The first cell was most likely (but not certainly) a random event. Which chemicals combined where. What chemicals happened when, what energy was present where. Probability determines when, how fast, and how often the necessary reactions occur.

This isn't very good--I don't know a thing about abiogenesis, so I can't give an exact link on probability. But for the most part, certain things have to happen for other things to happen. There exists discreet odds for this to occur for all events.

Monkeybot

I've heard, and I am by no means an expert, that the first "life" was not necessarily a cell but a self-replicating strand of RNA. So the complexity factor goes down a notch.

I'm also curious about viruses, which have some characteristics of life, but not all... I don't know if viruses are a step in evolutionary history, but it seems like they're a good example of *possible* transitional steps. Or am I totally wrong?

yguy

Then why hasn't someone tried my suggested experiment: breaking an amoeba down to the molecular level, applying various environmental stresses to it such as might have existed at the time the first cell came to be, and seeing how long it takes to reconstitute itself?

Absolute nonsense. To say anything is governed by probability is just a pseudo-scientific way of saying you don't know WHAT governs it, but you don't care to admit it.

DrLao

Because amoebas didn't evolve under the conditions that existed when life first appeared? Because it took billions of years for amoebas to evolve?

lpetrich

Viruses are not likely to be relics of the earliest organisms, since they are dependent on their host cells' replication and assembly mechanisms.

They are two main possible origins:

* Degenerate cells. These would originate from intracellular parasites, bacteria and protists that live inside of their host cells. Parasites are anatomically simplified compared to their free-living relatives, because utilizing their hosts' capabilities spoils them, and parasitic cells are no exception. In this scenario, viruses have simply gone further, throwing out most of their internal mechanisms.

* Mobile genetic elements. There are numerous examples of these. Many bacteria can inject each other with "plasmids", little loops of DNA, and many organisms have "retrotransposons", little bits of DNA that make copies of themselves elsewhere in a genome. The human genome contains something like 500,000 copies of a retrotransposon called "Alu" -- 5% of the genome, about as much as all the recognizably-functional parts (genes and gene-regulation regions).

So all that's necessary to make a new virus is for a plasmid or a retrotransposon to induce its release inside of a little bubble.

lpetrich

As to the first cell, there has been some interesting research into that subject. Brian Davis in his paper Molecular Evolution before the Origin of Species estimated the origin times of 10 important proteins from the complexity of the biosynthesis needed to make their amino acids (simpler = older). The oldest one is "ferredoxin", which is an electron-transfer protein that is important in biosynthesis.

The ancestral ferredoxin was not adapted to sticking to a cell membrane or some protein complex, but instead was adapted to sticking to a mineral surface.

So it could be older than cells, a relic of a time when Earth life was like Haeckel's Urschleim phase.

This is consistent with Gunter Wachtershauser's picture of the origin of life as having taken place in hydrothermal vents, where hydrogen sulfide would react with iron, contributing hydrogen to carbon dioxide, thus forming organic molecules. This formation would become cycles like the Krebs Cycle, which would become the foundation for later cells' biosynthesis.

Somewhere along the line, however, RNA originated; neither Davis nor Wachtershauser have a good picture of how RNA or some RNA precursor had originated, though it was most likely some byproduct of these protobiochemical cycles.

But once it got started, it made copies of itself, creating the "RNA world", with RNA as both informational molecule and enzyme. A world which has several vestiges in present-day organisms.

This RNA had a taste for using other molecules as cofactors to assist it, cofactors that included amino acids. The amino-acid-cofactor assembly system eventually became the familiar translation system, and these amino-acid cofactors eventually became the entire enzymes.

Discrete cells got started after that; of Davis's proteins, those adapted to residing in cell membranes are younger than ferredoxin.

Amino-acid biosynthesis was gradually built up, and as this occurred, new amino acids "stole" codons from older amino acids.

DNA is a latecomer. Its synthesis enzymes are relatively young, but still present before the youngest ancestor of all present-day life.

Jesus Tap-Dancin' Christ

So, quantum mechanics is pseudo science? There goes the last 100 years of nobel laureates. Glad you could point that out.

Your DNA is governed by random processes. Why did you get he DNA you did from your father? RANDOM CHANCE, nothing more.

On a small enough scale, everything is random. We just see the statistical average of all results.

yguy

Perhaps you could be so kind as to take issue with what I actually said, since I said precisely what I meant - nothing more, nothing less.

Allow me to test your thinking, if you will.

If a pebble is dropped from atop a building on Earth, the distance d that it falls in time t, neglecting air resistance, is governed by the equation:

d = 1/2 gt^2, where g is Earth's gravitational constant.

True or false?








The correct answer is "false". Why?

Hint: the validity of the equation is irrelevant.

yguy

OK, but SOME organism did. How come we can't throw some raw materials together and duplicate what happened in the beginning?

Really? I thought the earth was only 4 1/2 billion years old. That would be cutting things a bit close, wouldn't it?