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This argument from the article meshes with my own thinking: The reason there have not been new forms of life originating on Earth, evolving alongside our micro

This argument from the article meshes with my own thinking: The reason there have not been new forms of life originating on Earth, evolving alongside our microbial cousins, could be that there is simply no room for a newcomer. In a system where every niche is filled with more advanced life looking for a next meal, even if a new form of life got started, it would not last long. There are a number of related issues regarding life, including LUCA and the emergence of mitochondria in eukaryotic cells, which suggest to me less that a single ancestor existed at that time than that that ancestor's descendents out-competed (or out-lucked) all other comers, and that once established other contenders for life simply couldn't find an available niche. Oh, and bad news for the fans of LGM / panspermia, at least in the "absence of evidence" sense: Population statistics indicate that the microorganisms originated from terrestrial contamination during the sample preparation stage rather than being indigenous to the asteroid.

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throwthrow4567 2y ago
this argument is one of the many lesser known incredible insights from Darwin's diaries!
uticus 2y ago
> which suggest to me less that a single ancestor existed at that time than that that ancestor's descendents out-competed I’m not smart enough to get this. Why would it decrease the emphasis on a single ancestor? The suggestion of competition among descendants isn’t contrary to that, neither does one take from or add to the other.
- dredmorbius 2y ago
  The distinction: there's a frequent understanding or expression that a single common ancestor means that that was the only individual alive at a time, or that its ultimate dominance was evident immediately or very shortly afterwards. Neither of these is the case. Nor, strictly, is the notion that the individual (or its descendants) were necessarily evolutionarily fitter than others. In the first instance, a common ancestor may have come long before some evolutionary chokepoint or contest in which their gene line ultimately survived whilst others died out. And in some cases (mitochondria), the alternative gene lines didnt' die out. Non-mitochondrial organisms are the prokaryotes, and they have continued (for many hundreds of millions of years) to live alongside their mitochondrial-bearing eukaryotic cousins. Bacteria are AFAIU largely prokaryotic. Even some eukaryotes have lost their mitochondria, though it seems that all eukaryotes had mitochondria at some point in their evolution. But where it comes to complex organisms, overwhelmingly mitochndria-bearing cell forms out-compete those which lack same. As for evolution, that which is expressed in genomes requires a threefold process: variation, selection, and inheritance. That is, whatever leads to the eventual survival of a particular line of descendants must be based on an inheritable characteristic: stronger muscles, lower metabolism, superior coordination, robustness to environmental variance, faster development within a given niche, mate-selection advantage, etc., etc. (And note that what works in one selective environment may well not be suited to another.) Other selection events are not directly heritable, or are far less heritable. Getting smacked by an asteroid is an example, though there are features (e.g., being an aquatic life-form, small body size) which may increase survival odds after a major impact. Other natural disasters (earthquakes, tsunamis, volcanoes, wildfires) etc., are to an extent random and arbitrary in what they kill, though there may be specific adaptations which increase survival odds after such events. Even given this, many species will see a fairly arbitrary selection of extinction and survival, often with little prediction for a future repeat of such an event. Which individuals survive, or don't, is pretty much a wash. Those are still selection events, but don't convey any heritable advantage. The possibility that some common ancestor or LUCA is at least in part arbitrarily determined seems high to me.
abracadaniel 2y ago
This creates a new doomsday scenario for alien bacteria. Not that it could infect us, but that it could be much more efficient than existing life in some fundamental way that can’t be stopped.

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throwthrow4567 2y ago

this argument is one of the many lesser known incredible insights from Darwin's diaries!

uticus 2y ago

> which suggest to me less that a single ancestor existed at that time than that that ancestor's descendents out-competed I’m not smart enough to get this. Why would it decrease the emphasis on a single ancestor? The suggestion of competition among descendants isn’t contrary to that, neither does one take from or add to the other.

dredmorbius 2y ago
The distinction: there's a frequent understanding or expression that a single common ancestor means that that was the only individual alive at a time, or that its ultimate dominance was evident immediately or very shortly afterwards. Neither of these is the case. Nor, strictly, is the notion that the individual (or its descendants) were necessarily evolutionarily fitter than others. In the first instance, a common ancestor may have come long before some evolutionary chokepoint or contest in which their gene line ultimately survived whilst others died out. And in some cases (mitochondria), the alternative gene lines didnt' die out. Non-mitochondrial organisms are the prokaryotes, and they have continued (for many hundreds of millions of years) to live alongside their mitochondrial-bearing eukaryotic cousins. Bacteria are AFAIU largely prokaryotic. Even some eukaryotes have lost their mitochondria, though it seems that all eukaryotes had mitochondria at some point in their evolution. But where it comes to complex organisms, overwhelmingly mitochndria-bearing cell forms out-compete those which lack same. As for evolution, that which is expressed in genomes requires a threefold process: variation, selection, and inheritance. That is, whatever leads to the eventual survival of a particular line of descendants must be based on an inheritable characteristic: stronger muscles, lower metabolism, superior coordination, robustness to environmental variance, faster development within a given niche, mate-selection advantage, etc., etc. (And note that what works in one selective environment may well not be suited to another.) Other selection events are not directly heritable, or are far less heritable. Getting smacked by an asteroid is an example, though there are features (e.g., being an aquatic life-form, small body size) which may increase survival odds after a major impact. Other natural disasters (earthquakes, tsunamis, volcanoes, wildfires) etc., are to an extent random and arbitrary in what they kill, though there may be specific adaptations which increase survival odds after such events. Even given this, many species will see a fairly arbitrary selection of extinction and survival, often with little prediction for a future repeat of such an event. Which individuals survive, or don't, is pretty much a wash. Those are still selection events, but don't convey any heritable advantage. The possibility that some common ancestor or LUCA is at least in part arbitrarily determined seems high to me.

abracadaniel 2y ago

This creates a new doomsday scenario for alien bacteria. Not that it could infect us, but that it could be much more efficient than existing life in some fundamental way that can’t be stopped.