I always wondered about that. 
They do to mine, and I’m an evolutionary biologist in training. Look at our own planet. From an objective point of view, multicellularity is an evolutionary long shot. The vast majority of life on this planet is now and always has been single-celled. There are two major groups that have developed complex multicellular structures: plants and animals. Of the two, plants have completely failed to develop every one of your list of “basic things”. We’re very biased to think that our way of life is the one that makes the most sense.
Now, one could argue that if we’re talking intelligent space traveling aliens, that presupposes some of these things, and that’s a valid point.
I suspect we’ll all evolve second thumbs and huge, bat-like eyes for texting on 2 inch screens.
There is an old, obscure TV movie that I’ll spoilerize, just in the 0.0001% chance somebody actually wants to see it and somehow gets a tape of it:
There was a TV movie from 1994 called Official Denial, starring Dirk Benedict and Parker Stevenson, that had the abductor-aliens really being future humans so genetically modified that they can no longer reproduce. The abductions are part of an effort to regain that ability.
Yes; you’re right that it’s a huge bias, but this was in fact my supposition. Once we assume intelligent life, the conditional probability of those certain features becomes more favorable. I don’t think trees are going to be building any space ships any time soon. (Unless. . . :eek:)
Some features are more probable than others. Cephalization and bilateral symmetry are nearly a given for large, active life forms. However, number of limbs and sensory organs is much less determinate; there are a great range of these in animals. While vertebrates have four limbs, arthropods may have six, eight, ten, or many; mollusks may have one, eight, or ten. Arthropods have two large eyes but several additional small ones; ancestral vertebrates had a third (parietal eye) that has since been lost.
Walking bipedalism is perhaps the most unlikely feature. While hopping bipedalism has evolved independently many times, walking bipedalism may have evolved only twice: among the dinosaurs and in humans.
What is the basis of this assertion?
True, but I can’t help but notice that the larger terrestrial animals unfailingly have four limbs. Has there ever been so much as a six-legged rodent? 
I don’t have any biological expertise, but this seems to make a kind of sense to me: the larger you are, the more costly additional mass is in terms of structural integrity (and probably energy consumption). So if you’re going to have muscular legs, there seems to be a pressure against a larger number than is necessary. One is probably insufficient for a vertebrate. Two, of course, would require bipedal control. Three is out if we’re assuming bilateral symmetry. It seems to me that four is the smallest number that will do the trick for a terrestrial animal and, barring the profusion seen in small animals such as insects and other bugs, it also seems that more than four would add substantially to structural and metabolic cost while not providing a scalable benefit.
Maybe, given the fact that there are lots of many-legged critters and have been for a long time, a better way to put it would be that there seems to be a limit on how large animals with more than four legs can evolve to be. Does this make any sense, or is it perhaps just coincidence that we don’t have any eight-legged cows?
Unless I’m just talking out my ass here, that seems like a good reason to suspect that intelligent aliens might also have four limbs.
That’s pretty interesting, actually. Given that so many of these things seem to follow symmetry, it’s always a little surprising to hear about the exceptions. Like the fact that opossums have thirteen nipples. I mean, seriously?! :dubious:
In a way, aren’t we all from the future? I’m a different person now than when you started reading this thread. I’m in the future compared to before.
The problem is, of course, is that all large terrestrial animals on Earth belong to a single evolutionary line originating from the first vertebrates to emerge on land. I don’t see that there was any compelling reason for these early land dwellers to have had four limbs instead of six. So modern large land animals may simply all have four limbs because of developmental or evolutionary constraints due to their ancestry rather than mechanical or energetic ones. Given the skeletal structure of the first land vertebrates, it would have been very difficult developmentally to add additional pairs of legs, even if these would have been beneficial.
Likewise, most small terrestrial animals belong to another evolutionary lineage, the arthropods. These are characterized not only by having six or more pairs of legs, but also an exoskeleton. And it is the exoskeleton that provides the constraint on how big terrestrial arthropods can be. Arthropods have a modular body plan that allows for easy duplication of segments and hence additional pairs of legs. So the two facts that arthropods are small, and that they have multiple pairs of legs, are not necessarily evolutionarily linked.
All the hopping bipeds I can think of are descended (AFAIK) from walking bipeds.
But it’s very late.
None of the mammalian hopping bipeds are descended from walking bipeds. All mammalian hopping bipeds (kangaroos, kangaroo rats, jerboas and gerbils, springhaas, jumping mice, etc.) are descended from walking quadrupeds or possibly hopping quadrupeds.
Birds that hop are descended from walking birds. However, all birds belong to a dinosaurian lineage whose ancestral members were walking bipeds.
My personal opinion is that any alien life we eventually find will surprise the hell out of us, but until we have data, it’s all speculation.
I can’t speak for Brandus’s sources, but you can trace the progression quite clearly in early science fiction, in which speculations about future human evolution along the same lines as Rafe Hollister’s were a common theme, and where depictions of “advanced” aliens often followed the same reasoning.
From there, it’s possible to trace the way that popular fictional images of aliens have influenced the aliens encountered by UFO “contactees”, and how their accounts have then fed back into fiction.
So you’re from the past, you whacky time-traveller, you.
I’d say that makes you from the past.
I KNEW it!!!
Keep in mind that the current popular image of the skinny grey-skinned hig-headed humanoid is not the only public concept of what aliens look like. The ‘greys’ are a fairly recent idea. There have been other popular concepts of what aliens look like in the past, and there are other competing ideas right now. I think shapeshifting reptilians are still being claimed by some.
Yes, yes. I forgot about kangaroos. :smack:
Smaller than the (future) dinosaurs, sure, but the Arthropleura was estimated to reach up to eight feet in length. That is as big as a lot of African plains predators.
There seems to be a popular notion that evolution has “momentum” - that you can plot traits on a graph and extrapolate them into the future. I remember being told as a kid that humans will eventually have only four toes because our toes have been shrinking.
I personally believe that no species will achieve interstellar travel unless they modify themselves non-evolutionarily, through genetic engineering. Even more likely (or rather, less unlikely) is that they will develop AI probes capable of remaining functional after centuries or millennia of travel.