Saturday, June 21, 2014

Delivering the Mail

Ann Gauger, a biologist at Evolution News and Views has a brief but fascinating piece on the cellular "postal system." She points out that cells need a directed delivery system to make sure that proteins are distributed to the locations in the cell where they're needed. This transportation system is very complex and features at least three types of molecular motors that carry cargo throughout the cell. One of these motors, kinesin, is described in this short video:
Different "cargos" have different destinations. Gauger explains:
Some cargos go to the growing front end of a migrating cell. Others must travel the length of a neuron's long axon to get to their destination. And some have to be exported outside the cell. As an example, epithelial cells have three distinct surfaces, top, bottom, and sides. Each surface needs a different set of molecules delivered to it. Without this differentiation, tissues and organs made of epithelial sheets could not form or function properly. In embryos, special "determinants," either protein or RNA, have to be delivered to the right location in the developing egg or embryo. Once again, without these determinants the body plan of the nascent embryo is disturbed.
These facts raise some inevitable and extremely provocative questions:
How is all this coordinated? How does the cell know where to deliver its products? What is the address system? Where precisely is the map that matches all these addresses?
The cell acts as though it's an automated distribution system, but how is this system programmed? What part of the cell does the programming? It doesn't seem to be DNA since DNA codes for proteins, that is, DNA codes for hardware, not software.
This system is present in most if not all eukaryotes. In fact, it appears to date back to the first eukaryotes. Let's consider how this system might have come about. All parts are necessary for it to work. To return to our metaphor, if the postman just started transporting things at random, what benefit would that be? Yet address molecules are of no use without a postman, or without paths to travel on. Thus directed transport requires a complicated set of interacting parts, each of which is essential.

Without these motors and their interacting proteins, migrating cells wouldn't have the materials they need to move forward. Axons would die from lack of mitochondria and/or they would send signals very inefficiently. Epithelial cells would have their bottoms and tops confused. And embryos? A mess.

Oh, and I haven't mentioned that both kinesin and dynein (another transporter molecule) are essential for chromosome movement and spindle formation during cell division.
In other words, this distribution system must have evolved before cells developed the ability to reproduce, which means that this highly complex system evolved before there was evolution. Gauger concludes with this:
Amazing. Hard to sort out how it happened, isn't it?
Yes, it is. It's especially hard to sort it out if one is committed to a naturalistic explanation (i.e. an explanation in which only material, physical causal factors are allowed) for how it all came about. The naturalist account of the origin of such systems reminds me of the Queen in this exchange with Alice in Lewis Carroll's Through the Looking Glass:
Alice laughed. "There's no use trying," she said: "one can't believe impossible things.

"I daresay you haven't had much practice," said the Queen. "When I was your age, I always did it for half-an-hour a day. Why, sometimes I've believed as many as six impossible things before breakfast."
Carroll would have a lot of fun with today's Darwinian naturalism.