I wish I had gotten up several hours earlier now.
"Missing links" do not exist as they are so often imagined, and not just because of the self-contradictory term used to describe them (as in, if you find one then it no longer counts, no longer being "missing". Yes, I have encountered this argument before).
Consider this: every species that has ever lived has been a transitional species. Except, perhaps, those who died out and ended their lineage. But since every species is affected by their selection pressures, they will always change over evolutionary time. This raises the interesting problem of defining a species, because its like deciding when black ends and white begins on a grey scale when you've never actually seen either colour before.
That is not so much a problem when discussing prehistoric species, because all we have are brief snapshots of a lineage's evolutionary time, in the form of fossils. Because fossils are individuals, and therefore discrete entities in the otherwise continuous evolutionary gradient, defining one as a distinct species is rarely an issue.
But that means that, having arranged fossil species in an evolutionary arrangement, there are lots of gaps. Again, we're marking of points on a "grey" scale, but a grey scale with billions of primary colours. If there were no gaps, we'd be up to our armpits in fossils, and I, for one, would be extremely suspicious. And probably dead. But I digress.
So, each one of these fossils are transitional forms between their immediate ancestor and their immediate descendants. If they had any. So it is hopelessly unhelpful to refer to them as such, so when we talk about transitional forms we usually mean those representing the transition between major habitat changes and the physiological changes that occur, such as land-dwelling ungulate to sea-dwelling cetacean. Or maybe just the transition of a particular body part that a fossil represents, such as fish fin to tetrapod limb. And by Odd, do we have lots of those.
However, a property of discrete snapshots of a continuous process is that you can never expect to fill in
all the blanks, however useful and accurate a picture your snapshots put together. Another shameful argument I have encountered, upon presenting an important transitional fossil, is that I now have
two missing links, which was technically true until I presented those as well. The predictable response was that I now had four missing links and was exponentially losing ground. Rather than continue down this path by presenting the others I had ready, I succinctly explained to them the significant difficulties of their logic.
I can present examples of those very transitional fossils I described. Of jawed, bony fish to early amphibians:
Osteolepis,
Eusthenopteron,
Sterropterygion,
Tikaalik,
Panderichthys,
Elpistostege,
Obruchevichthys,
Hynerpeton,
Acanthostega,
Ichthyostega,
Pholidogaster,
Pteroplax.
Of land mammal to whale:
Pakicetus,
Nalacetus,
Ichthyolestes,
Gandakasia,
Ambulocetus,
Himalayacetus,
Attockicetus,
Remingtonocetus,
Dalanistes,
Kutchicetus,
Andrewsiphius,
Indocetus,
Qaisracetus,
Takracetus,
Artiocetus,
Babiacetus,
Protocetus,
Pappocetus,
Eocetus,
Georgiacetus,
Natchitochia,
Dorudon,
Ancalacetus,
Zygorhiza,
Saghacetus,
Chrysocetus,
Ghaviacetus,
Pontogeneus,
Basilosaurus,
Basiloterus.
Can supply pictures later if they are requested.
The problem with fossils are that they are incredibly rare. The sheer number that we have found and are still yet to find is testimony only to the sheer liveliness of planet Earth. But we know why we don't find more than we do, geology predicts that it is a rare occurrence, and never mind all those buried deep in the Earth's crust, under the sea or in other inaccessible locations. Consider that we have eleven fossils that have been identified as
Archaeopteryx lithographica (there is some debate over whether some belong in different species, even genera), several of which are only feathers. In the millions of years that
A. lithographic existed, there would have been billions of individuals born and died. Yet only eleven fossilised specimens have been found. That is a reduction by a factor of one hundred million, and many other species have even less fossils. Rare? You betcha.
Though some microscopic organisms produce trillions of individuals at the same moment in time, particularly planktonic organisms. As a result we have a much more comprehensive fossil record of those people have had the patience to study. See
here the transitional forms from
Globigerinoides trilobus to
Orbulina universa.
Models of evolution have yielded evidence that some novel features emerge and refine themselves so quickly (evolutionarily speaking) that it is not surprising that there are no fossils at all. Especially when the part is soft and does not normally fossilise. The eye has evolved at least 40 times independently of each other, and there has been enough time in any evolutionary lineage for it to have evolved from scratch 1,500 times over. A two-dimensional (for feasibility) computer model of the evolution of an eye from skin (Nilson and Pelger, 1994), using all the laws of physics and biology required to model evolution and light, produced a functioning fish eye in less than 400,000 generations. That is, if you'll excuse the pun, a blink of an eye.
[linkage]
Remember that a transitional form is not half a creature, but a whole creature in and of itself. It is easy to sea what a feature in a fossil will turn out to be, because we know its evolutionary history. But look at yourself. How much of you can you spot on its way to becoming a new feature? Because, assuming we are not an evolutionary dead-end, much of you is.
And, as is quite obvious from the fact that 99.9% of the Earth's species are extinct, so too are most of the Earth's transitional species, present company excluded of course. We don't expect to find one of our ancestors up and about today, they died out as advantageous genes emerged and spread through their population repeatedly until they either smoothly transitioned into a new species or another population that did came and out-competed them all. Though we do see living representatives of transitional species, those who share that species with us as a common ancestor and have retained some of those landmark features to this day. One of many famous examples are the Sarcopterygii, the lobe-finned fishes, such as the coelacanth and the lung fish. Another major group are all of the amphibians, all of the fish, in fact, every species has retained features from their ancestors and represent them to this day. That is the whole point I've been trying to make.
Oh, and transitional forms from earlier primates to modern humans?
Sahelanthropus tchadensis,
Orrorin tugenensis,
Ardipithecus ramidus,
Ardipithecus kadabba,
Australopithecus afarensis,
Australopithecus africanus,
Australopithecus garhi,
Australopithecus bahrelghazali,
Homo habilis,
Homo erectus.
Missing Link
And I'm pretty sure that's been found aswell, just in fossil form.
