For example, bilateral symmetry. It's so common amongst animals that it's easy to think it's universal, when there's no real reason it should be. (I recall vividly being startled when I first ran across H. P. Lovecraft's "Great Old Ones" -- who had five-way symmetry.) What's likely is that a very long time ago, one of our successful ancestors was bilateral, and passed that characteristic down to its descendants -- which include the majority of Kingdom Animalia.
So if we ever find alien life, there's no reason to suspect that it will share some of these probably-arbitrary characteristics with terrestrial life. Still, there are a few features that are significant enough advantages that it's likely to be found in other living things, wherever and however they evolved. One of these is cephalization -- having the important organs, including the central nervous system and sensory receptors, near the anterior end. Having your eyes and nose near your mouth makes a great deal of sense from the standpoint of finding food, and it's pretty likely to be a feature that shows up again and again. (Consider flight -- such a great adaptation that it's evolved independently at least seven times in Earth's history, in birds, insects, bats, colugos, flying squirrels, sugar gliders, and pterodactyls.)
Locomotion itself is one of those abilities that is so useful that it's likely to show up wherever life occurs, but it's one of those things that's so universal we tend not to think about it, or even be aware there are exceptions. (In fact, when I got students in my introductory biology classes to brainstorm for characteristics they thought were true for all living things, "able to move" was the most common wrong answer.)
This comes up because of a paper that was published in Nature last week, my awareness of which I once again owe to my sharp-eyed friend Andrew Butters of the brilliant blog Potato Chip Math. In it we learn about a fossil that seems to be the earliest direct evidence we have of locomotion in an animal. The fossil, which has been dated to around 540 million years ago, is the trail of a critter named Yilingia spiciformis ("spiky creature from Yiling"), about which the authors, Zhe Chen, Chuanming Zhou, and Xunlai Yuan (of the Chinese Academy of Sciences), and Shuhai Xiao (of Virginia Technological College of Sciences) have the following to say:
The origin of motility in bilaterian animals represents an evolutionary innovation that transformed the Earth system. This innovation probably occurred in the late Ediacaran period—as evidenced by an abundance of trace fossils (ichnofossils) dating to this time, which include trails, trackways and burrows. However, with few exceptions, the producers of most of the late Ediacaran ichnofossils are unknown, which has resulted in a disconnection between the body- and trace-fossil records. Here we describe the fossil of a bilaterian of the terminal Ediacaran period (dating to 551–539 million years ago), which we name Yilingia spiciformis (gen. et sp. nov). This body fossil is preserved along with the trail that the animal produced during a death march. Yilingia is an elongate and segmented bilaterian with repetitive and trilobate body units, each of which consists of a central lobe and two posteriorly pointing lateral lobes, indicating body and segment polarity. Yilingia is possibly related to panarthropods or annelids, and sheds light on the origin of segmentation in bilaterians. As one of the few Ediacaran animals demonstrated to have produced long and continuous trails, Yilingia provides insights into the identity of the animals that were responsible for Ediacaran trace fossils.So what this represents is not just the dawn of motility, but the dawn of bilateral symmetry, and Yilingia may have been one of the earliest animals that had both. It might not be our direct ancestor, but certainly was a close cousin to whatever was, and many of the features we now see in virtually all animals were locked in around that time.
It's awe-inspiring to look at this simple little fossil, the tracks of a critter that marched its way on the seafloor half a billion years ago, at a time when there was not a single thing living on the land, when the continents were bare rock, sand, dust, and dirt as far as the eye could see. And even more amazing to realize that this innovation -- the ability to move -- was passed down through all that time, refined in a thousand different ways, and is the direct ancestor to our ability to walk, run, crawl, and jump.
So far from feeling demeaned by our connections to our primitive ancestry, as the creationists would frame it, I feel exalted by it -- we are linked in an unbroken chain of relationships to every living thing on Earth, and everything we can do, every structure in our bodies down to the molecular level, is directly due to inheritance that stretches back to the very first life in the primordial seas.
And if that's not a mind-blowing thought, I don't know what is.
This week's Skeptophilia book recommendation is pure fun: science historian James Burke's Circles: Fifty Round Trips Through History, Technology, Science, and Culture. Burke made a name for himself with his brilliant show Connections, where he showed how one thing leads to another in discoveries, and sometimes two seemingly unconnected events can have a causal link (my favorite one is his episode about how the invention of the loom led to the invention of the computer).
In Circles, he takes us through fifty examples of connections that run in a loop -- jumping from one person or event to the next in his signature whimsical fashion, and somehow ending up in the end right back where he started. His writing (and his films) always have an air of magic to me. They're like watching a master conjuror create an illusion, and seeing what he's done with only the vaguest sense of how he pulled it off.
So if you're an aficionado of curiosities of the history of science, get Circles. You won't be disappointed.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]