well, most of them don't, but they would if they were fertilized. It would look like this

Interestingly, pineapples almost never reproduce by seeds. Even in the wild, most of their reproduction is vegetative. Why? Their huge, spiny fruits are adapted for being eaten by the extinct megafauna of South America. Ground sloths, Gomphotheres, etc. When those animals went extinct, pineapples and their relatives lost their usual means of dispersion and had to limp by on occasional lucky seed spread and vegetative growth...until humans started growing them.

There are three basic ways to do this.

For long-lived vines, bushes, and trees, it's done by cuttings. You find a rare mutant plant that produces good fruit with no seeds, then take cuttings from it. You root those in soil or graft them on to roots, and you essentially make a bunch of clones of the original. This is how it works for bananas and grapes.

For annual plants, it's done by hybrids. For example, seedless watermelons are made by hybridizing two strains that produce infertile watermelons with few seeds. It's a bit like breeding mules.

Finally, in some circumstances you can get seedless fruit by preventing fruit from being pollenated. This is how pineapples are kept seedless, for example, and it's why Hawaii was a major pineapple producer. There were no native pollinators that would pollenate the fruit, resulting in seed free pineapples.

With humans specifically, a lot of it has to do with dispersal and competitive niche exclusion.

Competitive niche exclusion means that you can't (usually, there are some exceptions) have two species occupying the same biological niche. A niche is a way of making a living...the foods you eat, the places you nest, the times you are active, etc. Other hominids were similar to humans, and modern humans have a very broad niche (we eat a lot of different things, live in a lot of different places, etc).

So it's not surprising that there's no other surviving hominids where humans are...you'd expect us to push them out of their niches. Earlier hominids seem to have narrower niches and so could survive alongside each other in some cases. And often, animal species occupy narrow niches that allow them to avoid competing, for example similar species of fish may specialize in living in different parts of a lake. So you get a lake full of several species of sunfish, for example, where one eats snails on the bottom, another eats bugs in the shoreline plants, and another eats plankton in the middle of the lake.

The other relevant factor is dispersal. Humans are very good at dispersing. Of course, you can fly around the world in a jet today, but even 10000 years ago we had walked or boated almost across the whole planet. And people didn't stop moving once they got to new places, people kept moving around between most of these populations. Most species aren't this good at dispersing, so you get one species here and another similar species there, and you wind up with a bunch of similar species in different parts of the world, originating from isolated populations. People just move around too much for this to happen.

Humans aren't unique, there are other species where there are no other similar animals for various reasons, but this why humans wound up this way.

There are some very speculative far future scifi ideas for mining the sun, it's called starlifting.

This is not the sort of thing you'd use to solve a helium shortage, though! It'd be like mining ice from Pluto to chill your drink.

I mean, you can build things out of wood, and as long as the wood doesn't rot that represents stored carbon. But you can't solve climate change just with wooden buildings, if that's what you mean.

>If we restored it to how it actually was, how is that a projection?

Because we do not, and can not, ever really know how it actually was. When the older reconstructions were done in all white marble, that's how people at the time thought it was...just like if we did it today, we may do it how we think it was. But even though we know more, we don't know everything. Constant decisions large and small will have to be made, and those will reflect modern ideas. It's just inescapable.

> probably as old as humanity itself.

Eh, well, it's probably slightly younger than however long people have been building stuff, heh

No, that was a direct response to your comment. A restoration of the parthenon with bright colors would still be a projection of our current ideas onto the past. It might be a more accurate projection, but it's still just a projection not the actual historical thing.

>So I'm aware that, most notably, humans and cows suffer quite a debilitating condition if they partake in cannibalism, as Kuru and mad cow disease from prion infection.

I think it's important to understand exactly how Kuru happened.

Specifically, it happened in a group of people who engaged in ritual cannibalism of members of their own group who had died. Someone in that group had a spontaneous case of CVJD. They died, presumably of the disease, and were eaten by group members...specifically by women and children. Men occasionally at meat, but never organ and brain tissue. And nearly all cases of Kuru were in women or children. People who ate infected tissue and became infected themselves died, were eaten, and passed the disease along.

So it wasn't just cannibalism that allowed kuru to spread, it was the specific circumstances of cannibalism. First of all, a rare case of CJVD had occurred in the population to kick the whole tragedy off. The people who were eaten had died of natural causes, so people who had advanced stages of kuru were themselves eaten and able to infect others. The people being consumed were group members, so the disease could form a continuous chain.

In other circumstances, Kuru would not have been as likely to propagate. For example, if this was cannibalism where enemies killed or captured in battle were eaten, there'd be less chance for the disease to spread. A person killed in combat would be less likely to be suffering from the disease in the first place. If the disease did spread from the first individual to those eating them, it would be less likely to get a second chance to spread (Because those warriors would have to themselves be taken by enemies that also practiced cannibalism and eaten). And if the tissue being eaten was flesh and not brain and nerve tissue, spread would also be less likely.

This plays in to the spread of the disease in other mammals as well. It's exactly what happened in cows, for example. Older animals that had the disease were ground up and their organ meat was fed to many other cattle, who were themselves ground up and refed when they died. That made for a continuous chain of transmission. In contrast, using your example of cannibalism in the wild of young animals, they are less likely to have a spontaneous prion disease in the first place. And even if it does, the chain of infection would not persist because the adult that ate the offspring would be unlikely to themselves be cannibalized, especially by more than one individual (which is required for the disease to actually spread).

TLDR

To actually spread prion diseases by cannibalism, it's not enough for cannibalism to occur. The individuals which contract the disease by cannibalism must themselves be cannibalized, and by multiple individuals, and so on in a chain of transmission.

That said, cannibalism does also expose you to other, more ordinary diseases. It's also often risky, since adults of the same species are usually dangerous prey. And you also run the risk of harming your fitness by eating a relative.

It's less about adaptive value or not and more about body plan.

Your basic tetrapod body plan is pretty lizard-like (in turn, it's very basically a fish with four legs stuck on). The key thing about this body plan is that the main axis is horizontal. Makes sense, after all the ground is horizontal and that's what you are moving across.

Most tetrapods, heck, most vertebrates keep this basic orientation (for exceptions, see seahorses). Swimming, flying, walking, the body tends to be held horizontally. And this goes for bipedal movement too, which is actually pretty common. If the hind legs are longer than the front legs, and the animals has a big tail (like most tetrapods), it's pretty easy to get up on two legs. Tends to happen in lizards when they run, they are essentially just popping wheelies. Loads of dinosaurs also went around on two legs, with the body horizontal and the tail out behind for balance.

But this doesn't really work with most mammals, which tend to have piddly little tails and long front limbs. Which is why you rarely see any sort of bipedal mammal.

Now like I said, most vertebrates go around horizontally. What's an exception? Stuff in trees. Climbing, hanging from branches, that often puts the body vertical. And requires good balance too. Various primates will go vertical and even bipedal in trees, running along branches and doing that sort of thing, or swinging and leaping around upright.

When we get to apes, you have no tail at all, so there's no hope of going around horizontal-bipedal like a normal tetrapod. But Gibbons like to spring around in trees, run along branches, and walk upright quite a bit when forced onto the ground. Apes that spend less time in trees need to get around more efficiently. Chimps and gorillas seem to have adopted knuckle walking, hominids seem to have improved the original upright stance for efficient use on the ground.

But it's only the odd confluence of lack of a significant tail and preexisting history of upright orientation that makes upright movement plausible in hominids.

As for penguins, you'll note they also lack significant tails and (unlike most birds) have the feet at the very rear of the body to improve swimming efficiency. Which also leaves them stuck with no other options.

>Lack of selection

This is a common misconception. People think natural selection works like this: Individuals which don't survive to reproduce are selected against, and individual which do survive to reproduce are selected for.

In that case, you would expect an expanding population to be experiencing little selection, because most individuals are surviving to reproduce, right?

But it's not correct, because what natural selection actually selects for is individuals who reproduce the most. There aren't just two buckets, where an organism is either in or out. If a trait results in the production of, say, 10% more offspring on average, selection will favor it.

In a growing population, traits which enhance reproduction on average will spread, and that's natural selection. And it will happen more efficiently too. In small populations, natural selection is countered by drift. Basically, a trait that's beneficial might not be selected for, because the individuals who have that trait might happen not to survive for some other reason. Chance might just be against them. But a big population reduces the effect of chance (and therefore drift) for the same reason that it's much easier to roll a 1 on one dice than on ten dice all at the same time. And as you note, a larger gene pool also means more variation to draw potentially beneficial mutations from in the first place.

> means many unfit genes propagate

Also, "unfit" genes don't exactly propagate in the absence of selection. Take a gene for something (say, a medical condition) that was previously harmful, and remove all selection on it (properly speaking this means the gene isn't unfit anymore, but we'll disregard that because I know what you mean). This won't actually cause the gene to propagate, instead it's expected to remain at the same frequency in the population. To actually spread, there would have to be some active benefit of having the version of the gene which (formerly) caused the disease. You could in theory have a slow growth of gene frequency due to mutations, but this is a slooooow process.

And remember, if the medical treatment allows people with the gene to survive, but they still have reduced fertility or are otherwise less likely to have kids than the average person, that gene will still be selected against.

Hmm, better pick up some before next November

Additionally, soon after the outbreak started China went through and eliminated a huge number of animals from wet markets and livestock farms...a reasonable response, but there was very little outside testing of those animals for viruses. They may well have wiped out the entire population of intermediate hosts.

I can't specifically answer, but I can give some extra information

Most plants are autotrophs, which means they can synthesize their own material from a set of basic elements and sunlight. This makes them a bit easier to grow in general.

Many fungi (and a few plants) are parasitic/symbiotic on the roots of other plants. Becuase of this, they are harder to culture. You can't just grow them in isolation, you need the organisms they depend on as well....or at minimum you need a replacement.

But with mushrooms it's even harder, because the mushroom is essentially the fruit of the fungus. And the fungus won't send up mushrooms unless environmental conditions are right. So not only do you have to grow the fungus, you also have to figure out how to get it to fruit.

And finally there's a question of demand. There has been progress made in culturing truffles, because truffles are super valuable. But most mushrooms don't have millions of dollars poured in to figuring out how to grow them, so nobody's figured it out yet.

The pan has to be hotter than the chicken to heat it effectively

Some fish have dual use lung/swimbladders, but a great many common fish have lost the respiration use entirely and the swimbladder often has no open connection at all remaining to the digestive tract. These fish often use other methods if they want to breath air, like the labryinth organ in bettas.

Even fish with totally closed swim bladders can absorb gasses from them into the blood...and push gasses into them from the blood. That's how they inflate and deflate them.

> As my understanding, lung did not evolve from gill

Correct. And as a side note, lots of people will tell you that lungs evolved from swim bladders, but in fact it appears that the reverse is true. Early lungs developed as a pouch off the digestive tract in early fish living in low oxygen waters, and only later did they develop into the specialized swim bladders that many fish have today.

https://www.ncei.noaa.gov/products/paleoclimatology/tree-ring

There's no evidence of this, but it's not just H. erectus that follows this pattern. There's a very clear transition of fauna between SE Asia and Australia that occurs in this region. The most famous division here is the Wallace line, which divides areas that were connected to the Asian mainland during the ice age from islands that remained separate, but early hominids actually dispersed a bit further, about to the Weber line. This is similar to elephants and some other large mammals, and implies they managed to cross short stretches of open water to reach these islands. But they apparently couldn't cross the larger gap between Timor and Australia.

It's worth noting that this dispersal happened during glacial periods when sea levels were lower and the geography of the area was quite different. You could walk all the way out to around Bali because sea levels were lower. And H. erectus and kin were all over south Asia and had been for more than a million years.

Really, it's not unusual to see that sort of spread. There's a fair amount of overlap between South Asian and African large mammals. When the Sahara is a grassland/savanna (which happens sometimes), large mammals with adaptable habits can disperse pretty easily between Africa and Asia, which is what you are seeing with hominins. But crossing the open ocean is much more difficult, and it seems that only modern humans managed it.

Here's a paper about hominins in SE Asia, which also has a map of sea levels and one of known hominin fossil sites

https://clas.uiowa.edu/sites/default/files/news-events/Larick-Ciochon-2015-Hominin%20Biogeography-ISEA-Evolutionary-Anthropology.pdf

There are several different contributing factors. One is that cities tend to get built on what are called depositional zones. These are places where sediments accumulate (as opposed to places sediment erodes from). People like to build cities near rivers and near the mouths of rivers....just the sort of places where rivers flood and dump a bunch of sediment. This naturally buries things over time.

The second is that, especially historically, rubbish built up in cities. There were no trucks to haul away rubble on a large scale. If a building collapsed, you just sort of knocked down the rubble and built a new house on top. Especially if your houses are made of stone or mud brick. This results in a layer on layer buildup of debris that can actually leave an artificial hill called a tell.

Also, there's a flat-earth levels of crazy conspiracy theory about how the whole world was flooded with mud about 100 years ago, wiping out evidence of some advanced globe spanning civilization and burying the lower levels of cities. It's not the truth, but you may run in to people talking about it so I figured I would mention it.

It's also thought that stars and planets form in fundamentally different ways, so an actual failed star like a brown dwarf should be different from a planet, even one of about the same size.

>are there any other examples of this in nature?

It's pretty common, but doesn't always happen.

>do ligers have the same issue in terms of reproducing?

Ligers are generally fertile. Note that it's not a hard cutoff, there's a range between hybrids that are almost always fertile and those that are almost always infertile. Plenty of species fall in the middle ground.

>is this why a chimpanzee and a human couldn't reproduce?

This has never really been thoroughly investigated, so we don't actually know if hybrids are possible. Hybrid incompatibility is about more than just differences in chromosome number, though. Specific adaptations can make egg and sperm incompatible (related species with overlapping ranges sometimes have adaptations like this to prevent hybrids) and sometimes the details of genetics can make hybrids nonviable.

I always wondered if this word had any relationship to macadamia nuts...turns out they are both named after different people with a last name of McAdam/MacAdam

Hydrogen and oxygen are common elements that were widespread during the formation of the solar system. Mix them together and you get water. Most of the hydrogen was lost off the rocky planets early in the solar system's history, but some of it got bound up in rocks ( along with a lot of oxygen) where it could be later released as water

Not an asteroid. One possibility (I'm not sure if it's currently in favor) is that asteroids and comets delivered much of Earth's water. We are talking not one or two, but very large numbers of them.

Another possibility is that most of Earth's water was emitted by volcanoes (which do emit water vapor).

And of course, a mix of the two is also possible.

Either route could potentially explain Mars' water.