[Note: this was going to be a post on obscure bizarre sponge biosynthetic pathways. They're bizarre all right, and obscure has never stopped me before, but the subject (sponges) isn't easy to fit into any paleo record. The following discussion is an old workhorse of mine, freely adapted from Karl Bloch's Blondes in Venetian Paintings, a book filled with many good things.]
The most common sugar on Earth is probably glucose, also known as blood sugar or grape sugar. It's found in blood and grapes, and pretty much everything else with a metabolism. It's ubiquitous in living organisms as an energy source.
A molecule of glucose has six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. That's proportionately the equivalent of one molecule of water (H2O) for every carbon atom. Perhaps you've seen the Mr. Wizard clip where the good wizard dumps sugar into the aggressive dessicant sulfuric acid, leaving behind steaming carbon foam. Hence carbo-hydrate.
In glucose, in water solution, five carbon atoms and one oxygen atom of the glucose molecule are often arranged into a hexagonal ring. It's not a flat ring, but staggered, a little like a lawn chair. Each carbon atom in the ring is connected to two other atoms not in the ring: a small hydrogen atom and a larger carbon or oxygen atom. Each carbon atom in the ring also has two slots for further bonding: an equatorial position, like a cup-holder, or an axial position, like a parasol.
This is important, because in glucose, all the larger atoms avoid the axial position, in favor of the equatorial position. As a result, glucose the most chemically stable of the six-carbon sugars. (Picture the parasols on the lawn chair crashing against each other and tipping it over.) This stability is likely why glucose was selected by evolution to be a ubiquitous energy source in the first place, several billion years ago.
There's an odd exception. Insects, while they use glucose in their metabolisms (everything does), use a different sugar in their
blood hemolymph, called trehalose.
Trehalose is a disaccharide: two sugar molecules connected together. In this case, both sugar molecules are glucose, the footrest of one lawn chair connected to the footrest of the other. This, incidentally, keeps the rings of the lawn chairs from unravelling and reacting with other molecules. Even the comparatively stable glucose will unravel and react with things over time; it's why diabetics are more prone to blindness and kidney failure. But trehalose won't. It can't.
Trehalose is also an excellent cryoprotectant and protectant against dryness. Physically, it seems to mimic the structure of the network of water molecules in their liquid phase. (Yes, liquid water has structure! it's always in flux, but there are consistent loose interactions between water molecules. It's like a rave.) The water molecules link themselves to the oxygens on the equatorial cup-holder positions on the the trehalose molecule. Even a small concentration of trehalose will maintain this loose network.
Vertebrates can't synthesize trehalose. Insects can and do, sometimes in large quantities, and its concentration in hemolymph is correlated with an insect's ability to survive cold or other extreme conditions. The honeybee, whose thermal regulation is taken care of by the hive, does not produce trehalose; the mountain stone weta, which lives in the dry cold of New Zealand, produces the equivalent of one hundred times human blood sugar levels in trehalose.
This use of trehalose as a shared metabolic innovation among the insects strongly implies that it's basal. This in turn suggests that the insect lineage arose under conditions where a protectant against extreme cold or dryness was necessary -- the late Ordovician glaciations come to mind, even though the first documented insect dates from the Devonian. How trehalose metabolism is regulated in the insects is still a mystery.
Finally, trehalose was rediscovered in an effort to chemically characterize Biblical manna. Yeah, that's right: manna from heaven? Bug juice.