Ever since John Maynard Smith & Eors Szathmary published their (1997) book on The Major Transitions in Evolution, the idea of "Major Evolutionary Transitions" (METs) has attracted the attention of biologists and astrobiologists alike. These are major *Major* evolutionary transitions; not normal, repeated macroevolutionary events like speciation for which there is an abundance of evidence and which we can study in real time; but seemingly unique, one-of-a-kind, never again repeated events. These "Major Evolutionary Transitions" (METs) have been variously called "critical steps" or "hard steps" in evolution. Just how many there are depends on one's definition and who you talk to, but the list usually includes things like the following:
(1) The origin of life (2) The origin of photosynthesizing bacteria (3) The origin of eukaryotes (4) The origin of sexual reproduction (5) The origin of complex multicellularity with cell tissue differentiation (6) The origin of human symbolic language/intelligence
These "hard steps" are thought to be extraordinarily improbable, one-time events in Earth's history. And some, like the origin of life on Earth, seem an impossible occurrence even within the age of the universe. As the researchers note, "it would take >>10^200 times the present age of the universe for a particular folding [of a 300 amino acid long protein] to occur, even assuming a sampling rate of 1 trillion conformational states per molecule per second and a volume of concentrated protein solution the size of Earth's oceans."
This problem is compounded by the presence of life so quickly on Earth, essentially as soon as the Earth was habitable (~4.3 Gyr) if reports of biogenic carbon in 4.1 Gyr zircon crystals are accurate. If true, then that means there is an extremely small window of time for the first *two* "hard steps"---the origin of life *and* also complex photosynthesizing bacteria---as the biogenic carbon discovered is indicative of carbon fixation in photosynthesis.
If this weren't difficult enough, there are some researchers (e.g., Raup & Valentine (1983), Multiple Origins of Life) who argue that (even if life did originate abiotically) the probability of life surviving is low unless there were multiple origins of life (they estimate 10 independent origins for one to survive).
The researchers note, "The transition to eukaryotic life also involves similar “chicken and egg” difficulties [as the origin of life], with uncertainty on how an archaeon acquired a proto-mitochondrion, since endocytosis requires complex machinery only present in eukaryotes. A second potential hurdle for eukaryogenesis was the survival of the first prokaryotic host with a bacterial symbiont. Without the protection of spliceosomes and a nucleus, the prokaryotic host would be disrupted by extensive intron transfer from the lysis of its symbionts, resulting in few functional proteins. The chimera cell would need to evolve these complex defenses faster than the mutation ratchet effect driving the (already tiny) population to extinction, which could have also required a rare specific outcome among a vast combinatorial space."
The researchers considered four "hard steps": the origin of life, origin of eukaryotes, origin of sexual reproduction, and the origin of human symbolic language in light of recent evidence that suggests there is only ~1 billion years left before the earth becomes uninhabitable due to the sun's increasing luminosity (vs. the 4 billion years once thought). This shorter window of time (~5 billion yrs) to achieve these evolutionary "hard steps" for an earth-like planet; combined with the fact that our sun is not a common type of star, and that planets orbiting red dwarfs (the most common type of star) do not seem habitable for life (which effectively rules out 75% of the Milky Way Galaxy); the researchers conclude that intelligent life must be rare in the universe. Importantly, the investigators indicate predictions of their model and what it would take to overturn their conclusions.
A recent 2021 study argues that "expected evolutionary transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude." (See, "The Timing of Evolutionary Transitions Suggests that Intelligent Life is Rare").
Ever since John Maynard Smith & Eors Szathmary published their (1997) book on The Major Transitions in Evolution, the idea of "Major Evolutionary Transitions" (METs) has attracted the attention of biologists and astrobiologists alike. These are major *Major* evolutionary transitions; not normal, repeated macroevolutionary events like speciation for which there is an abundance of evidence and which we can study in real time; but seemingly unique, one-of-a-kind, never again repeated events. These "Major Evolutionary Transitions" (METs) have been variously called "critical steps" or "hard steps" in evolution. Just how many there are depends on one's definition and who you talk to, but the list usually includes things like the following:
(1) The origin of life
(2) The origin of photosynthesizing bacteria
(3) The origin of eukaryotes
(4) The origin of sexual reproduction
(5) The origin of complex multicellularity with cell tissue differentiation
(6) The origin of human symbolic language/intelligence
These "hard steps" are thought to be extraordinarily improbable, one-time events in Earth's history. And some, like the origin of life on Earth, seem an impossible occurrence even within the age of the universe. As the researchers note, "it would take >>10^200 times the present age of the universe for a particular folding [of a 300 amino acid long protein] to occur, even assuming a sampling rate of 1 trillion conformational states per molecule per second and a volume of concentrated protein solution the size of Earth's oceans."
This problem is compounded by the presence of life so quickly on Earth, essentially as soon as the Earth was habitable (~4.3 Gyr) if reports of biogenic carbon in 4.1 Gyr zircon crystals are accurate. If true, then that means there is an extremely small window of time for the first *two* "hard steps"---the origin of life *and* also complex photosynthesizing bacteria---as the biogenic carbon discovered is indicative of carbon fixation in photosynthesis.
If this weren't difficult enough, there are some researchers (e.g., Raup & Valentine (1983), Multiple Origins of Life) who argue that (even if life did originate abiotically) the probability of life surviving is low unless there were multiple origins of life (they estimate 10 independent origins for one to survive).
The researchers note, "The transition to eukaryotic life also involves similar “chicken and egg” difficulties [as the origin of life], with uncertainty on how an archaeon acquired a proto-mitochondrion, since endocytosis requires complex machinery only present in eukaryotes. A second potential hurdle for eukaryogenesis was the survival of the first prokaryotic host with a bacterial symbiont. Without the protection of spliceosomes and a nucleus, the prokaryotic host would be disrupted by extensive intron transfer from the lysis of its symbionts, resulting in few functional proteins. The chimera cell would need to evolve these complex defenses faster than the mutation ratchet effect driving the (already tiny) population to extinction, which could have also required a rare specific outcome among a vast combinatorial space."
The researchers considered four "hard steps": the origin of life, origin of eukaryotes, origin of sexual reproduction, and the origin of human symbolic language in light of recent evidence that suggests there is only ~1 billion years left before the earth becomes uninhabitable due to the sun's increasing luminosity (vs. the 4 billion years once thought). This shorter window of time (~5 billion yrs) to achieve these evolutionary "hard steps" for an earth-like planet; combined with the fact that our sun is not a common type of star, and that planets orbiting red dwarfs (the most common type of star) do not seem habitable for life (which effectively rules out 75% of the Milky Way Galaxy); the researchers conclude that intelligent life must be rare in the universe. Importantly, the investigators indicate predictions of their model and what it would take to overturn their conclusions.