[warent]

When you're saying "Big brains" I assume you mean a higher level of cognition and awareness in some general sense, not a physically larger brain.

First I think you're getting the cause/effect of evolution backwards. For example, primates did not evolve hands so they could better grasp branches, but rather they better grasp branches because they evolved hands and so had a higher rate of survival.

So the question isn't so much "what causes complex intelligence" (your proposal being hands) but rather "why hasn't complex intelligence appeared for something like the gazelle family tree?"

It's an interesting hypothesis to say that perhaps it's because they don't have hands, but there are innumerable other factors that could also be an influence, I doubt it comes down to a lack of hands.

I'm not a biologist though so take what I'm saying here with a gigantic grain of salt.

[chrisweekly]

"primates did not evolve hands so they could better grasp branches, but rather they better grasp branches because they evolved hands and so had a higher rate of survival."

We agree that it can be difficult to distinguish causation from correlation, and in this case you may be right that the survival advantage conferred by better grasp of branches might have been a "bonus" or helpful side-effect, rather than an adaptive trait directly informing our ancestors' evolution. But I don't think it's so clear-cut or obvious.

[ramblerman]

This kind of nitpick seems a bit unnecessary, and I think it's particularly strong because people are contending with intelligent design nonsense.

When you talk generation to generation, the gradual tiny mutation each time did indeed allow primates to grab the branches a little better and so they had a higher rate of survival and the causation matters.

But when you talk about the end product (the hand), it is fair to say that primates (as the product of those millions of tiny iterations) evolved to better grab branches, imo.

[thomasfedb]

I think the distinction being made is between evolution and design. In evolution traits do not come about for a reason, they occur at random, but they persist if they are advantageous.

[mirimir]

Yes. But there's an illusion of intentionality. Because [pausing, looking at my hands] it's all just so bloody amazing.

But on the other hand, some of the kludginess is also bloody amazing. Spiders, for example. The gut passes through the "brain". So there's a tradeoff between "brain" size and eating. And so they employ external digestion, and suck liquid.

[downer56]

Well, a couple of things start to make sense about the kludginess, if you start with the single-cellular origin, and then pile up layers of specialized tissue, with some cells permanently buried within the organism, never to be exposed to an external environment.

The first part is that brains are just plain weird, and perhaps one of the weirdest aspected of any multicellular organism. More than half of all known life forms don’t have brains. So, brains usually aren’t a priority.

Second, the gut is often regarded as a brain-like organ in many vertebrates, and so, with arthropods and crustaceans, it’s interesting that the the two might be more tightly coupled. It probably cheapens the endocrine feedback loop.

Third, external digestion actually comes across as less freakish and uncivilized underwater. On land it has a messy, disgusting sensibility, but in the ocean it’s kind of on par with a smokey cooking fire at a camp site, especially when considered from a single celled animal’s perspective.

When you consider that crabs and lobsters are the ancestral relatives of insects and arachnids, the low prioritization of a brain, and externalized digestion start to look more and more normal.

[jhbadger]

Crabs and lobsters don't use external digestion but eat their food not terribly different from what we do. So it looks like spiders evolved their system later.

[mirimir]

Yes, they're not close relatives. Closer than us and them, but not that close. So yes, external digestion is largely a spider thing. However, crabs and lobsters do "chew" very thoroughly before swallowing. As do some spiders without external digestion.

In all invertebrates, "brains" (to the extent that they're present) are ring ganglia, with the gut going through the center. So arguably they all face that tension between brain development and the ability to eat. Vertebrates, conversely, have ~parallel guts and central nervous systems, so there's no conflict.

The thing with octopuses is that their ring ganglia are mode widely spaced. Having no exoskeleton helps with that, I guess.