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A reader emails to ask:
"Why must there exist a universal common ancestor?"
In other words, why do scientists believe that all living organisms today are descended from one ancestral population?
This is a really good question and the answer's not obvious, so:
THREAD
1/20
"Why must there exist a universal common ancestor?"
In other words, why do scientists believe that all living organisms today are descended from one ancestral population?
This is a really good question and the answer's not obvious, so:
THREAD
1/20
First, here is Ken's question in full:
"Why must there exist a universal common ancestor?
Suppose I manage to create new life in a flask. Neither you nor I will share its ancestry, nor will it share ancestry with ours..."
2/20
"Why must there exist a universal common ancestor?
Suppose I manage to create new life in a flask. Neither you nor I will share its ancestry, nor will it share ancestry with ours..."
2/20
"In nature maybe a later arrival would have been eaten for breakfast as you and Charles Darwin wrote, but only if it occupied the same habitat as the earlier one..."
3/20
3/20
"Going back to the origin, suppose life arose contemporaneously in several ponds or craters of comparable chemistry and ambiance, got tossed about and scrambled by storms, volcanic eruptions, meteorite impacts, or mudslides; exchanged genetic material laterally;"
4/20
4/20
"... and thrived, yielding parallel and possibly intertwined, evolved descendants. In that case, today's organisms do not necessarily share a common ancestry even though they might."
Again: it's a *good* question
5/20
Again: it's a *good* question
5/20
The short answer is that the Last Universal Common Ancestor, from which all modern living things descend, is not the same as the first life on Earth
Instead, LUCA may well have existed hundreds of millions of years after the first life
6/20
Instead, LUCA may well have existed hundreds of millions of years after the first life
6/20
The evidence that all modern organisms are descended from one ancestral population (one ancestral species, if you like) is strong
LUCA existed!
For one thing, all organisms share huge amounts of biochemistry
And tellingly, everything uses the same genetic code
7/20
LUCA existed!
For one thing, all organisms share huge amounts of biochemistry
And tellingly, everything uses the same genetic code
7/20
These similarities run deep
In 2016, Bill Martin's team tried to reconstruct details of LUCA
They looked for genes that were found in multiple microorganisms, even if those organisms were very distantly related
Such genes likely come from LUCA
8/20 https://www.nature.com/articles/nmicrobiol2016116
In 2016, Bill Martin's team tried to reconstruct details of LUCA
They looked for genes that were found in multiple microorganisms, even if those organisms were very distantly related
Such genes likely come from LUCA
8/20 https://www.nature.com/articles/nmicrobiol2016116
They found that 355 protein families were probably present in LUCA
That's a lot!
At this root biochemical level, all organisms are quite alike
This is very suggestive of shared ancestry
9/20 https://www.nature.com/articles/nmicrobiol2016116
That's a lot!
At this root biochemical level, all organisms are quite alike
This is very suggestive of shared ancestry
9/20 https://www.nature.com/articles/nmicrobiol2016116
But those 355 shared protein families also tell us something else
If we trust Martin's results at all, if we think they're even in the right ballpark, then LUCA was not a "simple" organism
300+ genes and proteins is more than some bacteria have today
10/20
If we trust Martin's results at all, if we think they're even in the right ballpark, then LUCA was not a "simple" organism
300+ genes and proteins is more than some bacteria have today
10/20
The odds that 300-odd highly specified things like genes would arise spontaneously and all at once are astronomically low
Instead, those hundreds of genes are clearly the product of a long period of evolution
LUCA, then, was not the first life, but its descendant
11/20
Instead, those hundreds of genes are clearly the product of a long period of evolution
LUCA, then, was not the first life, but its descendant
11/20
What was the first life?
The short answer is we don't know
But it must have been pretty simple, or it couldn't have formed by natural processes
The intricate complexity of something like a bacterium was achieved by a slow ratcheting process over millions of years
12/20
The short answer is we don't know
But it must have been pretty simple, or it couldn't have formed by natural processes
The intricate complexity of something like a bacterium was achieved by a slow ratcheting process over millions of years
12/20
This brings us back to Ken's question
He asks whether there could have been multiple origins of life on Earth, in different ponds or craters or other environments, and whether these separate lineages might have subsequently met and become entangled
To which I say: yes!
13/20
He asks whether there could have been multiple origins of life on Earth, in different ponds or craters or other environments, and whether these separate lineages might have subsequently met and become entangled
To which I say: yes!
13/20
Unless we accept that the formation of life was a 1-in-a-googolplex freak accident, we have to imagine that the processes that gave rise to life were fairly robust and repeatable
This seems reasonable: we find many "biological" chemicals like amino acids in meteorites...
14/20
This seems reasonable: we find many "biological" chemicals like amino acids in meteorites...
14/20
... which suggests these chemicals form fairly readily, given the raw materials and the right conditions
And there are experiments suggesting (for instance) that lipids can form cell-like membrane structures quickly and easily, again given the right conditions
15/20
And there are experiments suggesting (for instance) that lipids can form cell-like membrane structures quickly and easily, again given the right conditions
15/20
It follows that if conditions *somewhere* on the young Earth were suitable for life to form, conditions were probably suitable in multiple places
If life formed in a hydrothermal vent, there were plenty of those
If it formed in a pond, well, ponds are not rare either
16/20
If life formed in a hydrothermal vent, there were plenty of those
If it formed in a pond, well, ponds are not rare either
16/20
So there probably wasn't a single origin of life
If we time-travelled back 4 billion years or however long ago it was, we wouldn't be able to identify an individual pond that was the source of all modern organisms
There were probably lots of ponds
17/20
If we time-travelled back 4 billion years or however long ago it was, we wouldn't be able to identify an individual pond that was the source of all modern organisms
There were probably lots of ponds
17/20
But as time went on and the first organisms ventured outside their cradles, they presumably met their neighbours
Quite what went on then is utterly mysterious
Conflict? Cooperation? Some sort of merging? All three plus something else? Who knows
18/20
Quite what went on then is utterly mysterious
Conflict? Cooperation? Some sort of merging? All three plus something else? Who knows
18/20
In summary:
There probably wasn't a single origin of life, but many
However, all modern organisms are descended from a single population called LUCA, which came later
We don't know if LUCA outcompeted others, or if lineages merged to form it, or what
19/20
There probably wasn't a single origin of life, but many
However, all modern organisms are descended from a single population called LUCA, which came later
We don't know if LUCA outcompeted others, or if lineages merged to form it, or what
19/20
Thanks to Ken for emailing to ask the question: I hope this answered it!
There's loads more on how life might have begun in my book The Genesis Quest, so if you liked this thread maybe you should buy it
END
20/20 https://uk.bookshop.org/books/the-genesis-quest-the-geniuses-and-eccentrics-on-a-journey-to-uncover-the-origin-of-life-on-earth/9781474611411
There's loads more on how life might have begun in my book The Genesis Quest, so if you liked this thread maybe you should buy it
END
20/20 https://uk.bookshop.org/books/the-genesis-quest-the-geniuses-and-eccentrics-on-a-journey-to-uncover-the-origin-of-life-on-earth/9781474611411
