The walk of life

There are countless different actions we take every day that we do so automatically we're hardly even aware of how complex they are.

Take, for example, walking.  Walking takes the coordination of dozens of muscles, each of which has to contract and relax in exactly the right sequence to propel us forward and adjust for irregularities of the terrain we're navigating.  To avoid falling, we need to keep our center of gravity over our base of support, which is aided by adjustments in posture and such counterbalancing movements as swinging the arms.  But in order to do that we have to keep track of proprioception -- our sense of where our bodies are -- which is accomplished by a whole array of sense organs, including vision, the tactile sensors in the skin, and the semicircular canals -- the balance organs in the inner ear, which work a little like a carpenter's level.

And all of those -- the sense organs that keep track of what's going on and the muscles that use that information to contract and relax at the right times -- are linked by an astonishingly complex set of nerve relays and circuits coordinated by our brains.

All of that, just to get up and walk across the room.

The reason the topic of locomotion comes up is a paper a couple of weeks ago in Current Biology describing a single-celled protist called Euplotes eurystomus that has fourteen leg-like appendages -- and is able to walk.

The scientists studying Euplotes found that the appendages had thirty-two different configurations, which they called"gait states," and that somehow, the little creature was keeping track of which sequence of gait states allowed for the most efficient walking.  It turned out that the internal scaffolding of the cell, made of hollow threads called microtubules, created cross-links from each appendage to the others.  The amount of tension in the microtubules allows the organism to coordinate the movement of all fourteen appendages.

"The fact that Euplotes' appendages are moving from one state to another in a non-random way means this system is like a rudimentary computer," said Wallace Marshall, of the University of California - San Francisco, who co-authored the paper.

Euplotes eurystomus [Image from Larson & Marshall, UCSF]

All of this puts me in mind of the idea of irreducible complexity -- that there are structures in living organisms that would not work if all the parts hadn't been created all at the same time.  It's a favorite salvo of the young-Earth creationists and proponents of intelligent design.  The battle cry usually goes something like, "One percent of an eye isn't good for anything."  Richard Dawkins does a brilliant takedown of this entire argument in his tour-de-force exposition of the evolutionary model The Blind Watchmaker, in which he demonstrates that "one percent of an eye" -- a simple, light-sensing patch -- is indeed better than nothing, and that once you have that one percent, incremental gains in acuity can lead to the complex eyes now found in the animal kingdom.  (More fascinating still, the ability to sense light is such a powerful evolutionary driver that this process may have occurred, independently, dozens of times in different lineages.  For a wonderful overview of the evolution of eyes, check out this article from 2017 in the journal Nature.)

So locomotion, like vision, isn't irreducibly complex at all.  It's complex, yes; but hardly irreducible, because a single-celled organism is able to use chemical reactions and a network of microtubules to accomplish something very much like walking.  Our ability to coordinate motion has a history going back billions of years, during which time it has evolved into an action so complicated and intricate it's a damn good thing we don't have to keep track of it consciously.

Think about that next time you move your muscles.

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Walk of life

Even considering my background in evolutionary biology, there are a lot of things about the natural world that I take for granted.

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.

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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!]

Saturday science shorts

Because I am totally disheartened by the news, frustrated by the lack of critical thinking everywhere I look, and also because my blender exploded when I was making breakfast this morning and splattered orange juice and half-processed fruit over every square inch of the kitchen including myself, I am retreating to my happy place, namely: cool stuff in science news.

Let's start with a story from astronomy about something that is a near-obsession with me; the possibility of life on other planets.  This particular research involves the star system TRAPPIST-1, discovered last year and found to have not one, not two, but seven planets, three of which are in the so-called "Goldilocks Zone" (where the temperature is juuuuust right for water to be in liquid form).  Of course, that doesn't guarantee that water's there, just that if it was, it would be liquid, which scientists surmise would be a pretty good indicator of the likelihood of the probability of hosting life.

Now, researchers have found that all of the TRAPPIST-1 planets do have water -- in some cases, up to five percent of their mass.  So the three in the habitable zone might well be water-worlds.  All of which reminds me of the planet Kamino from The Phantom Menace, which otherwise was a dreadful movie, but I have to admit reluctantly that this part was cool.

Here's what we know about the TRAPPIST-1 system, although keep in mind that the illustrations of the planets are artists' renditions of what they might look like:

[image courtesy of NASA/JPL]

So that's pretty wicked cool.  The difficulty, of course, is that even if they did host life, it'd be hard to see that if the inhabitants had not advanced technologically to the point that they were sending out signals.  But even that hurdle might not be insurmountable -- as I wrote in a post a couple of weeks ago, astronomers are now trying to figure out if life is present on an exoplanet by the composition of its atmosphere.

Then, from the realm of biology, we have a study elucidating how those tiny jet fighters of the avian world -- hummingbirds -- maneuver as well as they do.

A group led by Roslyn Dakin and Paolo Segre of the Smithsonian Conservation Biology Institute of Ottawa examined hundreds of hours of high-speed video of hummingbirds in flight, looking at twenty-five different species and examining how they do their amazing aerobatics, including pivoting while in flight, hovering, and moving in an arc so narrow that it almost defies belief.  

The research took them to remote places in Panama, Costa Rica, and my favorite country of Ecuador -- the tiny nation that is host to 250 different species of hummingbirds, including the preternaturally beautiful Violet-tailed Sylph (Aglaiocercus coelestis):

Where I live, we have a paltry one species, albeit a beautiful one -- the Ruby-throated Hummingbird.  So it's no wonder the researchers decided to head south.

Another hummingbird researcher, Christopher Clark of the University of California-Riverside, has said that the new study is like moving from analyzing individual gestures of a ballerina to looking at how the moves fit together.  "Now," Clark says, "we're putting together the entire dance."

Last, some scientists at the University of Zurich have for the first time been able to see new neurons being formed in the brains of embryonic mice.  

Starting out by tagging 63 neural stem cells in the hippocampus, Sebastian Jessberger and his team were able to watch as the neurons grew outward and formed connections (synapses) with neighboring neurons.  What was most intriguing was that some of the new neurons had short lives -- perhaps acting as scaffolding for the developing brain and then self-destructing (undergoing apoptosis) when their task was complete.

Amongst these tagged cells, the red ones are the newest, orange next, and continuing through yellow and green (the oldest cells).

What is most exciting about this is that being mammals, it's expected that the knitting together of the embryonic human brain probably proceeds in a very similar fashion.  So what Jessberger et al. are doing might well inform us regarding how our own neural systems form.

So there you have it -- three cool new developments in the world of science.  Which has cheered me up considerably.  That's a good thing, considering the fact that now I have to go clean my kitchen, which I'm definitely not looking forward to.