I was talking to a friend recently about evolution and I mentioned how I would consider 4 things to be great leaps in evolution.
The origin of life 4 billion years ago.
The transition from prokaryotes to eukaryotes around 2 billion years ago.
The Cambrian explosion about 540 million years ago.
The tripling in human brain size about 1 million years ago.
Which begs the question, does anyone know the cause of each of these 4 events?
Like for event 1, my impression was that crystals formed near ocean vents, and some of those crystals made copies of themselves. This was likely the origin.
But I have no idea why 2 or 3 happened. Does anyone know?
For 4, supposedly climate change was the big cause. An ice age increased the demand for cognitive skills. But also the discovery of fire earlier made nutrition easier to get, as well as made food easier to digest. So our jaw muscles shrank allowing for more room for the brain to grow.
1 and 2 - certain configurations of organic molecules (eg. DNA) “reproduced” easily - the molecule splits down the middle and “attracts” the necessary amino acids to create two versions… apparently in the primordial soup the necessary building blocks were present. As a result, these molecules would come to dominate. Note that folded strings of DNA act as catalysts in building other proteins. For the molecules (DNA) where these proteins prove useful in ensuring the ongoing stability and reproduction of the source DNA molecule, those particular sequences would then also tend to dominate. Presumably some of these useful organics would disassemble other organics to use their building blocks for the predator molecule. Then some molecules would produce proteins/organics that stick to the exterior to protect from predator molecules, eventually becoming a membrane… but let in what is necessary to allow the core to reproduce. Followed by keeping a supply of “food” for the core DNA within the membrane. Then organizing the internal DNA into a more contained central location made the whole less vulnerable… so those that di so were more successful in the long run. In each case, the intermediaries fell by the wayside. In some cases, the one wildly successful example creates a whole new chain of progress.
So you can see that biochemical evolution is much the same as biological evolution - whatever helps to produce a more successful result in reproduction win, and over eons the result becomes incredibly complex. Think of it as dealing millions of bridge hands an keeping the ones that succeed as the start for the next game…
Once the specific mechanism of multicellular life and specialized cells evolved, the next step was for random results to create new forms of life. I’m going to guess part of the Cambrian explosion was the fact that there was no competition at first, so almost any form of life would succeed for a while. Every evolutionary niche was wide open. IIRC the initial weird life forms were bottom crawlers, so it probably also contributed to their variety of development that they did not spread quickly across the globe; a population that ended up in a separate area was unlikely to be invaded by competing types right away, had time to evolve to take advantage of the isolation. (Much as how Darwin’s finches, for example, were a result of being in an environment where there were no birds already evolved to exploit certain food sources - so the finches evolved into different species able to exploit different food sources.)
IIRC it was Stephen Jay Gould (among others, and also writing in “The Burgess Shale”) suggested the “winners” at the end of this life explosion were possibly determined as much by ecological and climate catastrophes and accidents as from actually “winning” the evolution competition. they were just the local winners, much as turtles and crocodiles are among survivors of the age of dinosaurs.
It’s a self-driving path. Luckily for us, it wasn’t one as fragile as the Pandas or Koalas reliance on a single diet. The just so story goes sort of like this - our ancestors came down from the trees to exploit food sources on the savannah, due to opportunities and climate change, hence more efficient upright walking (which also allowed carrying more food). As omnivores, we learned to exploit the leftovers from big game kills. Presumably they learned to exploit at first the small and weak and start doing the killing themselves.This allowed proto-humans to move toward hunting bigger and bigger game using cooperation. Cooperation works best with bigger brains and cooperative hunting, and leads to the tool use of more lethal weapons like point sticks and clubs. The bigger brain needs more protein (apparently our brain uses 1/3 of the food we consume.) So once headed down that path, it was difficult to “turn back”. The most successful hunters were the smart ones and pushed out the less successful ones.
What caused the sudden burst of brain evolution? Who knows. You could blame conceptual thinking, allowing for better planning of hunts. You could blame language, putting a strain on the amount of processing needed. Perhaps that’s when humans started to become very lethal to each other, and brain evolution was a by-product of learning to look over your shoulder to avoid being taken out by a competing tribe. Perhaps it became necessary when humans started to wander outside their original niche, and adapting to new environments meant needing to process an analyze more information. (Note that none of these are “they evolved this way because they needed…” it is more along the lines of “the ones best adapted to survive were the ones with the biggest brain… so their kids started from that and then again the best able to survive were…” Etc.)
At some level of generality, presumably these changes all burst forth because once they’d happened initially on a small scale, they represented such a huge competitive advantage that subsequent success was explosive.
As Darren Garrison said upthread, we’re not really sure.
The current theory for the Cambrian Explosion is that there wasn’t just one cause. It was a number of things that all came together. Here is my understanding of it (note - I’m an engineer, so this isn’t my field of expertise, I’ve just read a fair bit about it).
One factor was oxygen levels. Bacteria had developed the ability to photosynthesize, and over time, this dramatically changed the Earth’s atmosphere. Oxygen which had been tied up in compounds was released into the atmosphere, increasing oxygen levels significantly. Not only did this allow oxygen-breathing organisms to increase dramatically in size, but it also led to the formation of the protective ozone layer, which stopped incoming UV radiation from the sun from killing living things on the surface.
Calcium levels in seawater also increased, most likely due to volcanic activity. Living organisms could now use calcium to build bones and exoskeletons for protection. This led to an arms race of sorts, with creatures developing protection then predators evolving which could defeat these protections.
Some people believe that genetic material in general had progressed in complexity to the point where dramatic changes could be made to organisms much more easily, which, when combined with the arms races of sorts between predators and prey led to much more diversity.
Exactly how much each of these contributed and whether or not other factors were involved is still a matter of debate.
There are conflicting theories on the origin of life. Two prerequisites were (1) self-replicating molecule and (2) stabilizing matrix. The molecule and matrix associated with the earliest life may have vanished with little trace. Today’s ‘matrix’ is a cell enclosed by membrane, but the earliest matrixes may have been simple geologic micro-features.
And today, there is no self-replicating molecule! DNA requires complex proteins to help it replicate and those proteins aren’t even produced by DNA, but by the third chemical type in the chain, RNA. Long long ago, however, it is theorized that there may have been nucleic acids (not necessarily RNA) which could replicate, by serving directly as the catalysts for their own replication.
Another big evolutionary milestone was the development of sex(!), i.e. meiosis, which dramatically sped up the process of evolution about 1.2 billion years ago. Another milestone was life’s appearance on land, which happened not long after the “invention” of sex.
Thanks. As you no doubt discerned, I don’t know biochemistry - this was very interesting. But the point there is the same as mine - in the right environment, self-replicating molecules will replicate and changes that are improvements will be selected much in the same way as biological evolution works. There was nothing more magical and “impossible” about the initial emergence of life than about all the subsequent changes once life began.
There’s a concept called “punctuated equilibrium” in evolution that suggests that changes tend to happen quickly in response to changes in the environment; once a species is stable in their environment, change does not happen very much. Obviously something happened at the time that gave a serious advantage to bigger brains.
Why would you want to or need to mold genesis to anything?
probably needs its own thread, but seems a silly thing to do with something that doesnt need molded
on 4 - tripling of brain size, explanation became more complex following the discovery of Homo floresiensis [the smalll stature ‘Hobbit’ population from Indonesia].
Although they are likely to have been affected by island dwarfism as an evolutionary pressure, and their exact place on human evolutionary tree is debated, they show that the traditional connection made between brain size and cultural behaviour, specifically tool use is not mandatory. It may even be that things like tool use and fire free us from having to develop bigger brains to survive rather than being an essential driver of large brain adaptation.
Richard Fortey, the leading brainiac palaeontologist and trilobite expert also subscribes to the idea, which is as good an endorsement as I’d ever need. He has no need for any Genesis bits that others may have bundled up in the idea.
As a recommendation Fortey’s book Trilobites: eyewitness to Evolution is a cracker. A bit less hyperbolic than Stephen J Gould’s book on the Cambrian explosion, but an even better read to me. Lots on eyes.
Keep in mind that evolution is primarily an algorithm that optimizes towards local maxima. Out of the possibility space of all the ways you could organize matter to make a living system, evolution can only search the very nearby regions of this multi-dimensional space. This has to do with probability and the chance of survival with a given mutation.
What this means is a big part of our existence was due to luck. It was not inevitable. Evolution could have remained stuck on the dinosaurs or some other stable ecology until the star ran out of fuel.
