If it weren't for so-called "missing links," both the gaps in the fossil record and the gaps in living species due to the absence of descendents from extinct species, we wouldn't even have a concept of "species." There would be a complete continuum between one life form and the next, and no one would be able to place a stake in the record to say this is where one species ends and another begins. A hominid never gave birth to a homo sapiens, but over the millenia, there was enough difference between some of one generation and the next until, looking way back over time, we can see distinct differences with a whole lot of grey in between. A "species" and differentiation between species are human constructs - in life, there are no orders, no genus, and no species, just life.
Taking the trash out today, I marvelled at watching some birds across the street fly through the woods and around trees at astonishing speed, precisely veering in a split-second as needed to avoid colliding with limbs and trunks. Their minds must be working at remarkably fast rates to navigate the aerial terrain at that speed, I thought, far faster than my mind could process the hazards in my flight path (assuming I could fly).
What's it like to be a bird? Is all their thought that fast? And what constitutes "thought" for a bird, and does all life always rush by for them at that mile-a-minute rate? Watching them, it seems they rely on excellent wing-eye coordination, and I've rarely seen birds fly into objects other than glass buildings which they've mistaken for open sky.
I live with two cats, and have studied their minds - how they perceive the world, how they interact with me, with each other, with the physical world. They seem to be tunnel-visioned and don't rely on their eyesight nearly as much as we do, but still traverse a three-dimensional world, jumping up on and off of table-tops, dressers, and other furniture, scrambling around corners and immediately leaping up onto a desk, and so on. Their basic mode of getting around resembles the most complex parkour moves a human would ever attempt.
But they don't rely on eyesight to determine where to land and when to leap, and their remarkable hearing doesn't help them with inanimate objects. They rely on memory to tell them what's around the corner, what's below their next leap, what's on top of the table they are leaping up upon. So nothing pisses a cat off more than rearranging furniture and moving things around - their mental maps are suddenly of no use, and they have to learn all their jumps and leaps and dives all over again.
So birds seem to rely on their eyes and quick reflexes, and cats their memory and agility. In a famous essay, "What's It Like To Be A Bat?," Thomas Nagel noted how impossible it would be for us to understand the conscious experience of echolocution, and wondered if we could understand the conscious experience of any other species.
Going back to my opening statement about the arbitrariness of differentiating between species, if we drop away the concept of "species," Nagel's question takes on two different aspects. First, can we really know the conscious experience of anything other than the self? What it's like to be another human may be just as inscrutable as what it's like to be a bat. On the other hand, if all life's actually a continuum, and if we can come up with a reasonable, scientifically plausible conjecture of what it's like to be another person, then we should be able to extrapolate that to our closest species. But since the Neanderthal are extinct (probably due to our own aggressiveness), we may have to make the small leap to bonobos, or chimpanzees, or gorillas, or orangutans. From there, it's just another small step from apes to monkeys, and from monkeys to lemurs, and so on and so forth until we come to cats, bats, and birds.
Of course, the first thing this exercise shows us is that humans are probably the only living things that wonder what it's like being something else. As soon as we experience or understand the consciousness and experience of our nearest relatives, the process stops, because as far as we know, chimps don't wonder what it's like being a macaque.