Wow. Didn't it get hurt from the tree falling down on it?

>rapidly ascended

Doesn't sound very slothy.

breadth, chillth, coolth, dampth, dearth, depth, filth, health, height/heighth, illth, length, roomth, ruth, strength, troth, truth, sloth/slowth, warmth, wealth, width, wrath, wrength, youth/youngth

Which makes me think: i am surprised that is not an excessively used term in the startup / brotrepreneur world...

1. With their poor eyesight, if they were fast they'd half-blindly get into trouble and fall to their deaths or whatever, so it's kinda-sorta beneficial given that other constraint.

2. Being slow-by-design reduces calorie needs, meaning they can survive eating a smaller amount of stuff and stuff that other animals also don't want as much. This lets them fit into a low-competition niche.

3. Most nearby predators are looking for fast-moving targets, so being slow helps them avoid being detected.

It's because there's no singular reason for it. Evolution is blind (pun not intended), doesn't have motivation or reasoning. I think the article makes a good job explaining the set of evolutionary forces which likely pushed sloths into this direction.

> But doesn’t say why being nearly blind

The article mentions that this mutation already happened to the sloth ancestors, which lived on the ground. It's possible that it provided some advantage to those, or perhaps it didn't actually matter much (perhaps they were nocturnal?)

I imagine the author accidentally thought it was obvious. Having a slow metabolism is an evolutionary advantage in the same way not spending money is a financial advantage. It uses little energy and energy is usually the limit on what animals can do. If you don't spend much money, you don't need to earn much and can just exist. Being a high-earner-high-spender is probably a more efficient equilibrium, but spending nearly nothing works well enough to survive and if someone can spend less than they earn eventually they get wealthy.

If sloths don't do much, they don't need to eat much to balance out the energy expenditure. If they don't need to eat much, they can take less risks and tend to survive. Being slow is one aspect of their strategy of not needing to eat much.

Well they started early and had already arrived before all of us got here.

No reason to get in a hurry after that.

The human biceps is a good example of this. From a mechanical standpoint, where its insertion is like barely below the elbow joint, the bicep has extremely poor leverage, to the point where it needs to exert much more force than the hand does probably by a factor ~20-50. If it somehow went diagonally between the shoulder and the wrist, maximizing leverage, ignoring everything unpractical about this you'd be able to curl a small car engine. The downside is that a diagonal bicep would give us about 40 degrees of range of motion for the elbow and we'd be completely unable to throw things.

Big exception is jaw muscles (which are set up closer to that bicep thought experiment). Since they don't need to "escape" from anything and can sacrifice a bit of range of motion, they can afford to optimize for strength and endurance.

Evolutionary traits usually come with a tradeoff, and if a predator evolved to be able to catch sloths more easily it might make them less able to catch the juiciest prey, so overall it might be more worthwhile to focus on the other animals and leave sloths off the table, so to speak.

Most people think of evolution as a string of improvements, however you can get negative traits in a population from a large enough genetic bottleneck or spread by being next to a very useful mutation. So it’s not necessarily an advantage for them, it could have just been bad luck.

Alternatively, there could have been some selective pressure for it as a means of improving night vision or something, we just don’t know.