I know evolution is random and the whole Darwin-natural-selection thingy, but sometimes when you look at the process of evolution, for example in humans, it’s like evolution “decided” that we’d get bigger brains, lose hair, and go bipedal; and once it “decided” that we wouldnot go back.
I also know that enviromental pressures load the evolutionary dice in one direction
Is it simply a case that we only see the evolutionary “winners”?
I don’t see that at all. Where do you see direction in the fact that one species has bigger brains and walk on two legs? Dolphins have big brains and they swim. Human brains look for patterns in things whether they are there or not and that’s exactly what you are doing here.
Perhaps I’m not understanding what the OP means, but it may just be confirmation bias. We’ve all seen cartoons with variants on the old “fish crawl onto the dry land and turn into amphibians and then there’s a caveman in there and then something punch-line-y happens”. But of course life doesn’t have any “direction” of oceans–>dry land; whales and icthyosaurs and (to a lesser extent seals) have “gone back” from the land to the sea. The evolution of flight in birds seems extremely directed to us–the very essence of birds is flight–but flightlessness has re-evolved in birds multiple times. Evolution seems powerfully directed towards things like keen senses, yet cave-dwelling creatures often lose their eyesight. Evolution seems powerfully directed towards strong limbs, yet snakes (and independently some lizards) have lost their legs.
It only looks that way if you don’t look at the details. In our case, we are the only surviving member of our lineage. You don’t see all the side branches that became extinct. For one thing, we are not the largest-brained member of our lineage; that distinction goes to Neanderthals. Different components of the modern human anatomy evolved at different rates. For example, we became bipedal long before our brain size began to increase much beyond that of chimps.
This would only be true if environmental pressures are constant, which they never are.
We see in the present only the lineages that have survived until this time. If you look at any particular point in the past, you would be unable to predict which of the lineages present at that time would survive to the present. You can’t predict a priori which lineages will be winners and losers.
Pretty much what I was going to say; human evolution looks somewhat directional due to hominid species largely being failures. If one looks at a more successful family of species like felines or canines, you see an evolutionary bush, with many branches leading to many species; not a ladder leading to a single species at the top of it.
The lack of other surviving hominids may not be purely due to chance. There’s fair odds that H. sapiens may have killed off some of our cousins as it spread around the world, including H. neanderthalensis, H. floresensis, H. erectus, and the Denisovians.
Part of the illusion, I think, arises simply from the fact that we normally pay much more attention to humans, and then to other species largely to the extent that they are like us in some way or other. In fact evolution had produced, and continues to produce, many more species of insects, of all different sorts of plants, and of bacteria and viruses, than it has of hominids, or primates, or mammals, or even vertebrates. Evolution does not converge on us, it spreads out in all directions in teh space of biological possibilities. However, it is the vertebrates, and particularly the mammals, and particularly the primates, that you mostly hear about (in popular science books and TV shows, and even in college classes). This is not completely unreasonable: after all, it is naturally ourselves that we most care about and most want to understand, but it does tend to give a distorted perspective.
Darwin understood this problem, and wrote about the weird, outré, quirky things that nature puts forward. When you look closely, you can find a lot of things in nature that do not look directed at all.
Stephen Jay Gould alluded to that in the title of his book “Hens’ Teeth and Horses’ Toes.” These are two physical structures that are about as absurdly ad hoc as can be.
It isn’t enough to look at the really elegant parts of nature – eagle’s wings and wolves’ teeth. It’s when you look at the whole picture, you see the messy, sloppy, undirected and extremely unintelligent bits. These point the way to the truth.
Just because we didn’t “go back” doesn’t mean evolution is directional. Others obviously have gone back. Penguins evolved from flying ancestors “back” to ocean dwellers. Everything looks directional because we’re looking at what’s happened and draw lines to what’s there now, but as has been mentioned, there are a lot of “dead branches” and if we could draw lines showing every single mutation we’d see a lot more back and forth and sideways.
Evolution is directed (in the sense of: controlled) - it’s directed by natural forces, environmental conditions, competition, geography, chemistry & simple physics. What it isn’t, is directed (in the sense of: targeted or aimed). It is wrong to say “evolution is random”. It is not completely random, it is a mix of random and controlled things.
There are some senses in which evolution doesn’t easily allow you to “go back”, as it were (look at Dawkins’ Climbing Mount Improbable for a good overview of fitness landscapes in this context). For instance, I wouldn’t say penguins are a case of “going back” - they didn’t re-evolve gills and swim bladders and migrate their earbones back onto their jaws. They just adapted to a new environment for birds.
When we look back it seems a miracle that we reached “here” despite the plain truth that virtually every other creature who ever lived does not have any surviving descendants. Combined with our story-telling and pattern recognition tendencies it is inevitable that we view biological history as being guided to us as an end product.
There’s also a tendency with evolution for creatures to settle on some schtick and develop it further. A large part of the great apes’ schtick is that they’re more intelligent than most other animals, so it’s not too surprising that one particular lineage of apes took that schtick to an extreme. This isn’t really any different from some theropods specializing in biting things with a really big mouth, leading eventually to T. rex. From our point of view, evolution looks “directed” towards intelligence, and a smarter creature seems “more evolved” than a stupid one… But from a rex’s point of view, evolution is directed towards big jaws and teeth, and a more mawsome creature is “more evolved” than a small-mouthed one.