A little hard to explain that if you don't believe there's common ancestry.
That example shows up in just about every biology textbook ever written, but there are scads of others, including homology at the molecular level. A phenomenal discovery over thirty years ago shows that the proteins that make up the lens of the vertebrate eye -- the alpha-crystallins -- are related to a family of proteins called heat-shock proteins that protect our tissues from stress of various kinds, including temperature fluctuations. Apparently some time in a collective ancestor, the gene coding for a heat-shock protein acquired a mutation that gave it a new property -- transparency. Once that happened, it took evolution off in a completely new direction, a phenomenon that has been observed often enough that there's a name for it (three, actually; preaptation, preadaptation, and exaptation).
One long-theorized example of both homologous structures and preaptation is the collection of three little bones in our middle ear -- the malleus (hammer), incus (anvil), and stipes (stirrup). They act as sound-conducting devices, the vibrations being passed from the eardrum to the little bones, where they resonate at the same frequency and transfer those sound waves into the organ of hearing, the snail-shell-shaped cochlea. They're pretty critical; a friend of mine had a congenital defect in those bones, where as she got older the hammer progressively tilted away from the anvil, and the loss of contact was gradually robbing her of her hearing. Amazingly enough, microsurgery on this tiny (eight-millimeter-long) bone was able to correct the defect and restore 90% of her hearing, a testimony to the phenomenal advances medical science has made in the last decades.
Their position and shape clued in evolutionary biologists that as odd as it sounds, the three bones in the middle ear of a human are homologous to three much larger bones in the jaw of a reptile. You might imagine that given the delicate nature of these structures and the general paucity of fossils, it'd be difficult to find intermediate points in the lineage that showed the transition -- and indeed, that "missing link" (much as I hate that term) was absent for years.
Not any more.
Just this week, a paper came out in Science called, "Integrated Hearing and Chewing Modules Decoupled in a Cretaceous Stem Therian Mammal," by Fangyuan Mao, Yaoming Hu, Chuankui Li, and Yuanqing Wang (of the Chinese Academy of Sciences), and Morgan Hill Chase, Andrew K. Smith, and Jin Meng (of the American Museum of Natural History). It describes a little shrew-like early mammal called Origolestes lii, which lived in what is now China between 133 and 120 million years ago. This places it in the early Cretaceous Period, contemporaneous with a huge variety of dinosaurs (although still a good bit earlier than the Cretaceous usual suspects such as Triceratops and Tyrannosaurus rex).
The phenomenal degree of preservation of this little mammal allowed scientists to study the fine structure of the middle ear bones, and they found exactly what had been theorized -- that the three little bones in our ears had unhooked themselves from the jaw joint, moved, and gotten smaller, and Origolestes was right in the middle of that transition. The authors write:
The auditory bones, including the surangular [a bone found in the jaws of all vertebrates except mammals, homologous to our malleus bone], have no bone contact with the ossified Meckel’s cartilage; the latter is loosely lodged on the medial rear of the dentary. This configuration probably represents the initial morphological stage of the definitive mammalian middle ear. Evidence shows that hearing and chewing apparatuses have evolved in a modular fashion. Starting as an integrated complex in non-mammaliaform cynodonts, the two modules, regulated by similar developmental and genetic mechanisms, eventually decoupled during the evolution of mammals, allowing further improvement for more efficient hearing and mastication.So that's pretty spectacular.
Once again, evolution wins the day, not that there should be any doubt at all left. The evidence for the evolutionary model is abundant even if you discount the fossils entirely, but this kind of thing is just the icing on the cake, similar to the missing intermediate forms between land mammals and whales, which were found -- precisely as predicted -- in an amazing fossil bed in Pakistan in 1994.
Astonishing as it may seem, evolution rarely produces completely novel structures; it molds and repurposes what's already there for new functions. The tired old canard of finding an automobile in the middle of a field implying a manufacturer is woefully far off the mark; a closer analogy would be taking a car, and gradually modifying it part by part so that each change confers an advantage (or at least confers no disadvantage), and over millions of little transitions, ending up with a Boeing 747.
I'll end with the most direct statement on this topic ever made, a statement wholly supported by little Origolestes with its not-quite-jaws, not-quite-ears, from twentieth-century biologist Theodosius Dobzhansky:
"Nothing in biology makes sense except in the light of evolution."
Long-time readers of Skeptophilia have probably read enough of my rants about creationism and the other flavors of evolution-denial that they're sick unto death of the subject, but if you're up for one more excursion into this, I have a book that is a must-read.
British evolutionary biologist Richard Dawkins has made a name for himself both as an outspoken atheist and as a champion for the evolutionary model, and it is in this latter capacity that he wrote the brilliant The Greatest Show on Earth. Here, he presents the evidence for evolution in lucid prose easily accessible to the layperson, and one by one demolishes the "arguments" (if you can dignify them by that name) that you find in places like the infamous Answers in Genesis.
If you're someone who wants more ammunition for your own defense of the topic, or you want to find out why the scientists believe all that stuff about natural selection, or you're a creationist yourself and (to your credit) want to find out what the other side is saying, this book is about the best introduction to the logic of the evolutionary model I've ever read. My focus in biology was evolution and population genetics, so you'd think all this stuff would be old hat to me, but I found something new to savor on virtually every page. I cannot recommend this book highly enough!
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]