Another cool example of convergence

Okay, I just told ya about front-loading blood, right?  Actually, I brought this up over a year ago.  Well, let’s now take in the following recent study:

Front-loading Blood

Your body depends on a continuously moving stream of blood in order to stay alive. Why? Because it is the blood which carries the oxygen needed to fuel the electron transport chain in the mitochondria of all your cells. The continuous movement of the blood, thanks to the heart, coupled with the continuous supply of oxygen to the blood, thanks to the lungs, means all of the body cells have the ability to continuously generate ATP by their mitochondria. And that ATP is needed to run the variety of molecular machines inside the cells.

But the liquid portion of the blood, the plasma, can only dissolve and carry about 3% of the body’s oxygen demand. The other 97% of the oxygen must be carried by the blood transport protein, hemoglobin. Hemoglobin is composed of four amino acid chains known as globin, each one with a red pigment molecule known as heme embedded inside. The heme binds on ionized form of iron, which in turn is where the oxygen binds. Every red blood cell is packed with hemoglobin, thus oxygen.

What this all means is that your large, complex, multicellular body exists because of the globin protein. It is the globin, with its ability to bind, hold, and release oxygen that facilitates its existence.

That’s an Old Smell

Given that the membrane proteins TRPV2 and TRPM8 are used as sensors to detect “hot” and “cold,” respectively, would it be plausible that for such sensations to have been front-loaded?  After all, both sensors are highly conserved in all vertebrates.

Inspector Bunny decided to take a quick look and use the mouse sequence for these proteins to BLAST away.

Taste the Cold

Something of interest from a couple of years ago:

Several years ago, the specific receptors that allow us to detect heat were identified in nerve cells.  Closely related to the capsaicin receptor (TRPV1), TRPV2 is an ion channel that, upon activation by heat, allows positively-charged ions to enter neurons.  This creates a potential difference across the cell membrane and therefore an electrical current.  Given that these two receptors are closely related, it isn’t that surprising that exposure to capsaicin, the active ingredient in chili peppers, is sensed as “hot.”

Now a new paper published in Nature has shown that on the other end of the temperature spectrum, cold is detected by the same ion channel that is activated by menthol.  Known as TRPM8, the ion channel is activated both by menthol, a compound found in mint, and by temperatures below 26? C.  The researchers from UCSF, the Medical College of Wisconsin, and Yale have shown that isolated, cultured nerve cells that express TRPM8 react to cooling stimuli, but cells cultured from mice lacking TRPM8 do not.  Further, the mice lacking TRPM8 are much less sensitive to cold than their normal equivalents.

Cilia from the Telic Side

Given that all five of our special senses are built around cilia, how might we think about this fact from a teleological perspective?  Well, all I can say is that this fits quite well within two primary themes that I have been fleshing out over the last couple of years.

First, if we wanted to front-load evolution such that the sense of sight and hearing are more likely to emerge when metazoan-type life emerges, then we would expect cilia to be quite ancient.  And in fact they are – they are as old as eukaryotes themselves.  In fact, the last common ancestor of all eukaryotes not only had cilia (or flagella), but probably had the complex system for ciliogenesis (a process known as intraflagellar transport).

Of course, I have long known about the ancient state of cilia, so let’s push it some more.  Let us propose the original core function of cilia has always been sensory.  In other words, while most people think of cilia as motility structures, this may be more of a secondary function.  If the original function for cilia has always been sensory, then “the business end” of our five senses have always been there from the start of eukaryote existence.

Where It All Begins

So what do the special senses all share in common?  They all begin with cilia, the little hair-like projections that extend off the surface of cells.  When it comes to the sense of smell and taste, the cilia bind molecules and an electrical change in the membrane is produced.  When it comes to the sense of hearing and balance, the cilia are bent and an electrical change in the membrane is produced.  When it comes to the sense of sight, photopigments arrayed within the cilia absorb light and an electrical change in the membrane is produced.  Below is a figure of the three different cilia (shaded in green) tailored to detect changes in three different forms of energy:

Figure from Tomer Avidor-Reiss, Andreia M. Maer, Edmund Koundakjian, Andrey Polyanovsky, Thomas Keil, Shankar Subramaniam and Charles S. Zuker.  2004. Defining Specialized Genes Required for Compartmentalized Cilia Biogenesis Cell (04) 117:527-539.

We know these cilia are essential to our senses.

Special Sense Quiz

There are five special senses located in our heads: taste, smell, hearing, vision, and equilibrium (balance).  These senses provide the majority of information about our environment and together can detect changes in three different forms of energy – chemical, light, and mechanical.

So now it is quiz time.  There is something that all these special senses share in common.  Do you know what it is? Below the fold is a hint: figures that illustrate the core detection components of all five senses.

Adventures at BioLogos

A couple of weeks ago on BioLogos, biologists Darrel Falk and David Kerk posted a very nice essay on the evolution of the heart.  The main point of their essay is found in this paragraph:

So in the fruit fly, that which controls the “make-a-heart” switch is TINMAN. What controls the same switch in other organisms? The zebra fish is a little aquarium fish which produces transparent embryos. This transparency trait makes it the embryologist’s dream organism and thereby it has been widely studied. Fish, like fruit flies, have a heart. We talked about the structure of the fish heart in our last post. What controls the “make a heart” switch in zebra fish? You guessed it. Out of the hundreds of proteins that operate various switches in fish embryos, the one which controls the “make-a-heart” switch is almost the same as the fruit fly, TINMAN. In fact, when one puts one of the several zebra fish versions of TINMAN into mutant fruitfly embryos which lack their own TINMAN, one of the zebra fish versions will throw the switch, and cause the mutant embryo to produce a heart.² Zebra fish and fruit flies have been on separate lineages for over 500 million years, however each, despite hundreds of possible switches, still retain a switch that can only be activated by a variety of the TINMAN protein. So do other organisms like frogs and mammals have this same switch operated in the same way? Yes, all tested vertebrates, including humans make their heart in response to the TINMAN signal. It has changed a little more in mammals so the mammalian varieties are not interchangeable with the fruit fly’s TINMAN, but it is still the same gene which makes almost the same protein.

Once again, the concept of deep homology emerges to play a key role in evolution.  Yet Falk and Kerk did make one claim that I found to be misleading:

Going Green

Let’s take a closer look at some of the data patterns that emerged from the recently sequenced sponge genome. We are slowly getting to the point where we can begin to time the origin of complex genetic circuits essential for metazoan life.  For example, consider the figure below.  It represents the core elements of signal transduction pathways involved in cell-cell communication and coordinated growth:

From M Srivastava et al. Nature 466, 720-726 (2010)

Note the components are color-coded according to their distribution on the tree of life.  To help you visualize this, I used the same color-coded circles to identify the nodes below.

Another genome supports FLE

With more than 18,000 individual genes, the sponge genome represents a diverse toolkit, coding for many processes that lay the foundations for more complex creatures. These include mechanisms for telling cells how to adhere to one another, grow in an organized fashion and recognize interlopers. The genome also includes analogues of genes that, in organisms with a neuromuscular system, code for muscle tissue and neurons.

According to Douglas Erwin, a palaeobiologist at the Smithsonian Institution in Washington DC, such complexity indicates that sponges must have descended from a more advanced ancestor than previously suspected. “This flies in the face of what we think of early metazoan evolution,” says Erwin.

Charles Marshall, director of the University of California Museum of Paleontology in Berkeley, agrees. “It means there was an elaborate machinery in place that already had some function,” he says. “What I want to know now is what were all these genes doing prior to the advent of sponge.”

The analyses of Srivastava and her colleagues suggest that there was a crucial window, some 150 to 200 million years in duration, when the basics of multicellular life emerged. Nearly one-third of the genetic alterations that distinguish humans from their last common ancestor with single-celled organisms took place during this period. These changes would have occurred within our sponge-like forebears.

Here