Marcello treated us to a blog yesterday on the sponge genome sequence, and yes, when I told a friend I was going to write a sequel to a sponge blog, I got predictable cracks about squarepants.
In posting a response to Marcello, I noted that, splendid as sponges may be, unicellular creatures are endowed with most of the genes found in the sponge genome. Multicellularity was something of an afterthought, coordinated by genes that were already in place for other purposes in the unicellular world.
As it happens, I wrote a 13.7 blog back in March where I develop this same point: we eukaryotes (non-bacterial life) trace our ancestry back to a single-celled Most Recent Common Ancestor (MRCA) that inhabited the planet some 1.5 to 2 billion years ago. This MRCA encoded all the core eukaryotic “ideas”: how to make membranes with channels, how to regulate gene expression, how to engage in meiotic sex. These ideas then moved through evolutionary time into numerous radiations, with particular ideas becoming expanded and elaborated, others degraded and lost, in particular lineages.
In our particular lineage -- the multicellular animals -- the key distinctive idea turns out to be constructing a nervous system. Modern sponges don’t have nervous systems, nor do single-celled organisms, nor did the eukaryotic MRCA.
So how do you come up with a nervous system?
All modern nervous systems are made up of differentiated cell types, called neurons, that have the ability to excite one another, or excite muscle cells, by secreting hormones in specialized regions called synapses. The proteins that allow synapses to operate have been identified and exhaustively analyzed, as have the regulatory proteins that serve to tell certain cells in the early animal embryo that they should differentiate into neurons and produce such synapse-localized proteins. The sponge genome proves to encode many of these proteins.
So what is their function in the neuron-free sponge?
Elegant techniques have been devised to answer these kinds of questions, and the results are just starting to come in. Attention is focusing on so-called globular cells in the larva of the sponge, cells that express a subset of the synapse-localized proteins and a subset of the regulatory proteins that specify neuron-ness in animals. The globular cells protrude from the front end of the larva and are thought to serve as sensory cells, secreting their as-yet-unidentified contents in response to external stimuli.
The notion, then, is that there occurred what is called an exaptation: An ancestral creature with larval globular cells gave rise to two lineages, one leading to modern sponges that have retained the globular-cell idea, and the other leading to modern animals whose “proto-neural” globular cells went on to acquire the capacity to differentiate into full-fledged neurons.
But here we return to our foundational point. Secreting stored molecules in response to external stimuli is hardly the invention of globular cells or neurons. Many modern single-celled organisms in divergent radiations secrete all manner of substances in response to all manner of stimuli; the ciliate Tetrahymena, for example, secretes copious mucus when it is irritated. Moreover, many of the synapse-localized proteins and neuron-specifying proteins in the sponge genome are also encoded in the genomes of unicellular eukaryotes. Hence we are pushed all the way back to the eukaryotic MRCA, endowed with the idea of stimulus-induced secretion, an idea that was then passed along to the proto-globular cells in some pre-sponge creatures and then to the proto-neurons in some pre-animal creatures.
Stepping back, then, the big-picture view of evolution is not unlike the craftsman in his workshop (yes, could be a craftswoman). He has lots of materials at hand: pulleys and ropes, gears and wheels, bearings and bolts, tape and glue, switches and wires. With each project, some get used and some are left on the shelf; some are used as-is, others are tweaked, and some need major configuration to fashion the resultant product. French biologist François Jacob called the process bricolage, from the French word meaning to “fiddle” or “tinker,” conveying the concept of the patchwork quilt, of works created by putting together materials that happen to be available.
The craftsman analogy can of course be misunderstood as suggesting the existence of a Designer with a Plan, which isn’t the way it works. The tinkering takes place not during some weekend in the shop but over thousands and then millions and then billions of years. Countless combinations of this and that are discarded, not because of a decision made by some craftsman but because natural selection, that most exquisitely choosey of evaluators, has deemed one of the resultant widgets to be a non-starter and another to be promising, given the context.
Eukaryotic sponges and humans and daffodils and amoebae all have comparable numbers of genes, encoding similar kinds of proteins. The name of the evolutionary game is to come up with useful outcomes: the globular cells are likely as important to the project of being a sponge larva as neurons are to being a lemur. A clunky process has yielded mind-boggling adaptations and exaptations, again and again, because the materials are both time-tested and versatile and the stakes are, well, as high as life itself.