Intelligent Design advocates chose poorly when they selected the compound eye of vertebrates as one example of a complex biological structure that could not have evolved in stages. Even first-year biology students know about eye spots in planarians which are simple pigment patches at the front of the worm that are connected to their simple nervous system. They can only sense light versus dark, but that is enough for them to get by as planarians. There are many intermediate forms of eyes in the animal kingdom ranging from simple pits in the skin lined with receptor cells, all the way up to the compound eyes of animals with their cornea and lens arrangement. Fossils of trilobites that are over 500 million years old show they possessed eyes very similar to those of modern day insects. Our complex eyes clearly evolved from the simpler eyes in ancestral species, and the presence of the critical light-receptive pigment called “opsin” in all animals highlights this shared ancestry. The same molecule is used to sense light in worms, jellyfish, eagles and humans.
But what about the evolution of complex molecular structures, such as Intelligent Design’s perennial favorite, the bacterial flagellum? For details on how the flagellum most likely evolved stepwise from a bacterial secretory system, see:
Scientists are increasingly using genetics and molecular biology to dissect probable evolutionary steps in the formation of various molecular devices in animals, and one such recent study by Ken Kosik and colleagues has looked at cellular junctions in certain species of sponges.
Yes, sponges really are primitive animals, not just absorbent kitchen cleaning items. Sponges are among the simplest of multicellular animals, and they lack internal organs, including a nervous system. However, the mobile larval form of a species of sponge which has been studied extensively has been found to possess the majority of genes for making a critical part of neural synapses, even though the sponge larvae do not have nerve cells. Instead, they have cells called flask cells.
The flask cells of these larval sponges have many features of primitive sensory cells, including a cilia and secretory vesicles. Despite the fact that the larva have no nervous system, they nonetheless possess approximately 70% of the genes required to make the complex structure of neural synapses known as the “postsynaptic density” (PSD). PSD’s are the receptive part of a synapse which receives signals in the form of neurotransmitters released by other nerve cells. So if these sponges don’t have a nervous system, why would their larvae need so many genes associated with synapses?
The answer is that they appear to be using these genes to make signaling structures that are distantly related to neural synapses in animals. The genes in these sponges show a remarkable similarity to the related genes in animals that possess nervous systems, including the structural elements that hold the molecules into a functional array. The authors note that these proto-synaptic structures are not only likely candidates for the evolutionary stepping stones to synaptic contacts between neurons, they may represent prototypical cellular junctions in general which could have led to the development of many specialized junctions between cells found in later-evolving animals.
The take home message from such studies is that the same genes and molecules are used over and over again by different animals to perform many different functions, and that these simple building blocks can be mixed and matched in differing ways to produce increasingly complex molecular devices and organ systems. This derived complexity in no way undermines the notion of evolution, it fortifies the theory immensely. Rather than being another gap in human knowledge about evolution, molecular biology is demonstrating how very complex biological structures can evolve from simpler systems by making use of modular units that can be combined in many different ways, with each change making the system function more robustly. Eye spots are just fine for worms, but not for eagles, and yet the 530 plus million years of evolution between them provided innumerable opportunities for step by step improvements in vision.
Intelligent Design proponents don’t provide us with scientific data, they provide us with uninformed commentary and conjecture. Their arguments may work well with the uneducated public, but they are not based on scientific facts. The main underpinning of their arguments rests entirely on the concept of irreducible complexity. But Darwin is striking back with scientific data that shows how life is like a self-assembling Lego set, mixing and matching simple building blocks to make increasingly complex structures. When Darwin strikes back, he does so with great vigor and eloquence.