This could theoretically be about Mitochondrial Eve but in fact it’s about someone who lived many millions of years before her. But just to mention her in passing, this is how it goes. Mitochondria are ancient former bacteria who now live in the cytoplasm (jelly bit) of plant and animal cells, among others, and enable us to release energy from glucose efficiently. They still have some DNA and therefore have a genome. When a sperm fertilises an egg cell, its (his?) own mitochondria are usually abandoned and the only surviving ones are in the cytoplasm. Therefore, just as Y chromosomes are only inherited from fathers, mitochondrial DNA is only inherited from mothers – the cells and their mitochondria divide, passing on to future generations. They do change slightly with the passage of time, like all living DNA and even more so RNA, so they can be used to trace female lineages. My own female lineage is shared with most of the Libyan Tuareg, but that doesn’t mean they’re my ancestors so much as that my type of mitochondria used to be all over Europe and North Afrika but has disappeared elsewhere. North Afrikan humans have historically tended to be quite isolated because of the natural barriers of the Sahara, the Straits of Gibraltar and the narrow Sinai land bridge, so genes tend to linger there for longer. Anyway, mitochondrial DNA can be traced through its variation back to a woman living in East Afrika up to 230 millennia ago. This woman is of course everyone’s gran, although there would clearly have been other people around at the time who weren’t descended from her and she and they would likewise have been descended from a single woman in the more distant past, and so on. This is a bit long-winded but I just wanted to get that out of the way.
But we have much more distant grandmothers. Technically we could and kind of have traced them back thousands of millions of years to the LUCA – Last Universal Common Ancestor – living in the Hadean, soon after the formation of this planet. My focus today is on animals who existed in the final one percent of this planet’s history so far, and it comes out of the question “are we descended from monkeys?” The answer is to do with clades.
A clade is a group of organisms who are all descended from a single ancestor, that ancestor often being considered as an entire species rather than a single individual. Living humans constitute a clade of descendants from Mitochondrial Eve and also from Y-chromosomal Adam. Individual bits of genomes can also be thought of as clades, so although they overlap completely nowadays, we are all in a number of different clades, including all descendants of Y-chromosomal Adam and all descendants of Mitochondrial Eve despite the fact that there would have been countless individuals in the stone age and perhaps later who would only have been in one or the other of those. Cladism has taken over the practice of taxonomy, which is the science of classifying organisms. Before genomes were routinely fully sequenced, it wasn’t possible to classify them as precisely as they are today, leading for instance to the idea of edentates, a group which included anteaters, sloths and armadillos, but also the relatively distantly related aardvarks and pangolins. Nowadays that order has been retired because it turns out that the first three are so distantly related to all other placental mammals that they are more or less a sister clade to them almost like the marsupials, and aardvarks and pangolins are not closely related to each other at all either. Nonetheless I have a lot of affection for the old system because it did usually reflect morphological similarities which did reflect natural kinds in my opinion, just not natural kinds which reflected genetics as such.
A taxon (classification category for organisms) is a clade if it includes all individuals descended from a common ancestor. This is called a monophyletic taxon. The other two types are polyphyletic and paraphyletic. A paraphyletic group is one which excludes some descendants of a common ancestor, an example being the old idea of hominids as excluding chimpanzees, bonobos, gorillas and orang utan but including humans, and a polyphyletic group is what’s left once you’ve defined a paraphyletic one, so for instance the idea that orang utan, gorillas, chimps and bonobos exclude humans but form a natural group separate from us. Incidentally, I’ve oversimplified here because there are two species each of gorillas and orang utan, so in fact there are seven surviving species of ape. A particularly striking example of a polyphyletic group is the reptiles, which are of course snakes, lizards, crocodiles, alligators, tortoises, turtles, terrapins and the tuatara, which is not a “real” group because crocodiles and alligators are more closely related to birds than they are to any of the other “reptiles”. This always comes to mind when I consider the reptilian humanoid conspiracy theory, since this claims that we are ruled by alien reptiles when in fact in a sense reptiles themselves don’t really exist as such, so what exactly are reptilian humanoids supposed to be? A slightly more annoying consequence of cladism is that it wipes out the previous supercilious bit of pedantry where you “knowingly” claim that apes are not monkeys, removing that particular bit of masculinist discourse and information-hoarding, because in fact we are monkeys genetically speaking, and it’s this that finally gets me to the point of this post: omomyids. Well, almost.
Humans are in fact monkeys because humans are apes and apes are monkeys. Apes are a specialised development of the Old World Monkeys, i.e. the likes of macaques, langurs and baboons rather than capuchins, marmosets and spider monkeys, with various traits in common with them such as properly opposable thumbs, but lack tails and are more terrestrial due to their weight, and have wider bodies than other monkeys. Cladistically, we’re monkeys in a similar sense to being animals, and more specifically Old World Monkeys. There are also New World Monkeys. It would’ve been interesting if the New World Monkeys had evolved separately from non-simian ancestors, because apart from anything else it would increase the probability that humanoid aliens exist, and in any case it would’ve been an interesting and very close to home example of convergent evolution, like spiny tenrecs and hedgehogs or whales and fish, but in fact all monkeys form a clade. There is, however, an example of convergent evolution in the New World Monkeys in that their physically smallest taxon, including the marmosets, include examples who are physically quite similar to the prosimians found in the Old World who are not descended from the same ancestors, namely the likes of pottos and slow lorises. Something slightly similar also happened with apes, where gorillas seem more human than chimps and bonobos but in fact it’s the other way round, probably because gorillas and humans are both larger than chimps and bonobos. But in any case, humans, other apes and other monkeys are all descended from an older group which is almost completely extinct, but is today represented by tarsiers. Tarsiers are the most physically similar primates to the omomyids, who are our common ancestors.
Tarsiers, however, are rather specialised. They’re carnivorous and, being nocturnal, have very large eyes which dominate their heads. Because these eyes are so large, they cannot move them and instead have necks which can turn their heads almost all the way round like owls. They seem to have been around since the Eocene, over thirty million years ago, and I personally suspect they’ve survived longer than their relatives because they’re specialised in a direction which few other primates are, being for instance carnivorous. In the Galactic Association universe, incidentally, all primates native to the planet Athena are carnivorous and I’ve speculated that they’re descended from tarsier-like forms after this specialisation took place. There are arboreal predatory mammals elsewhere on Earth but none of them are primates.
If you take away the specialisations tarsiers evolved which seem to have preserved them for so long, you get a bush baby like animal who has smaller eyes, can’t turn her head most of the way round, is omnivorous and sometimes active during the day more than at night. This is an omomyid, and is our Eocene ancestor. Omomyids were, slightly surprisingly, native to North America (as well as Europe and elsewhere) although there are no longer any indigenous primates in North America apart from humans and those who have spread into Mexico from South America when it collided with its neighbouring continent less than three million years ago (and of course Sasquatches if they exist, which I myself doubt). This complementary distribution also happened to camels, who nowadays only exist where they originally didn’t. Although some omomyids did have smaller eyes, most of them did have rather large ones and therefore were likely to be nocturnal. There’s a rather unreliable principle that once a feature evolves or disappears, the descendants of the organism won’t retrace its steps, and this really doesn’t work very well but it probably does apply to the omomyids in the sense that we probably descended from the ones who weren’t nocturnal, one of whom lived in Texas. At the time, it seems that most of the land surface of the planet was covered in rain forest. The Eocene was one of the hottest periods of all in the history of Earth, and there were even rainforests in the Arctic at the time. Hence Texas was, like everywhere else, full of thick, impenetrable jungle.
Omomyids weren’t carnivorous. Their teeth suggest they ate fruit like many other primates. Incidentally, while I’m talking about teeth, the presence of canines is not indicative of an omnivorous or carnivorous diet, and like other primates, herbivorous or not, omomyids did have canines. In fact humans have smaller canines than most other apes, including practically herbivorous ones such as gorillas. Getting back to the point, it’s hard to say that humans are descended from monkeys, partly because we just are monkeys, but also because going back to the Miocene gets you a huge thicket of lines of descent, including more and less humanoid forms not all of which are closely related to each other or ancestral to us, and consequently we can say we are descended from monkeys, though it’s unclear which ones, but it’s a much stronger claim that all of those forms, including our ancestors, descended from omomyids, who were more like tarsiers than monkeys as we tend to think of them today.
They did, however, have a sister group, the adapids. These were less similar, being entirely non-simian, but are ancestral to the prosimians such as lemurs, bush babies and lorises. These shared their habitat with omomyids and used to be thought our direct ancestors, but it turns out they are real examples of convergent evolution with monkeys and are only similar to them because their lifestyles were similar. They also lived in North America, for instance what was to become Wyoming. Modern apparent descendants of adapids have a “grooming claw”. Whereas most primate digits have nails rather than claws, one finger is sometimes specialised to enable the animals to groom themselves. This was thought to be confined to the likes of lemurs, but it turns out some monkeys have them too and adapids sometimes have them or lack them, which was thought to be crucial to deciding whether they were our ancestors. In fact it seems that grooming claws evolve sporadically throughout the primates and don’t indicate any particular pedigree.
Pushing back further gets us to the common ancestors of the flying lemurs, who are like flying squirrels with a membrane between their limbs allowing them to glide between trees, and the primates – the primatomorphs. These used to be thought of as also being ancestral to the tupaias or treeshrews, who have also been considered primates in the past, but very recently these seem to have been revealed as being closer to rabbits and rodents. However, rodents are also fairly closely related to us, and at this point another example of irritating pedantry arises. It’s often said that rabbits aren’t rodents, and in fact this is true – hares, rabbits and pikas are all in the order lagomorpha, who unlike rodents are completely herbivorous and have four incisors in their upper jaw by contrast with the two of rodents (and I think always eat their own poo in order to finish digesting it, unlike rodents, but I might be wrong there). However, above the level of species there really is no firm definition of any category of organisms, and lagomorphs are closely related to rodents, so as far as I’m concerned it’s nitpicking, and another example of masculinist information-hoarding and gatekeeping signalling if that’s a thing, to point out that rabbits are not rodents. What is interesting about all this is that rabbits, rats, colugos, tupaias and humans are all in a relatively huge group of relatives called the euarchontoglires. We have little in common not also shared with other mammals, but we are unique among placental mammals in usually being born with a vermiform appendix – some marsupials also are, so it’s not a diagnostic trait. The earliest known euarchontan, and also the earliest known mammal with fingernails and toenails, was Purgatorius, who lived almost immediately after the extinction of non-avian dinosaurs. The problem, incidentally, with mammalian palaeontology of small mammals is that often they can only be identified by dental records because that’s all that survive. Hence you will see a lot of the oldest mammals referred to as something-odon or something-odonts, because that’s all anyone has to go on, initially, and this is true of Purgatorius, who was initially identified by bits of jaws with teeth attached to them.
The basic purpose of tracing our ancestors back this far was just to explain my reference to tarsiers in yesterday’s rant about veganism, i.e. who they are in relation to us. It’s possible to glimpse from their own ancestors who we were before the impact which ended the Cretaceous, and one of the interesting things about that is that not only do they lead us into the age of dinosaurs, but in the case of Purgatorius they might even have been around when it ended.