The burgeoning fields of molecular genetics, evolutionary development, and the study of genes revolutionized the understanding of evolution and began to allow great insight into not only what happened as fossil studies showed, but how evolution took place.
9. Study of Gene Changes-Effects leading to greater complexity—This discipline is a methodological approach to how transitional forms likely developed. An article in National Geographic Magazine, November, 2006, pp 114-135 by Carl Zimmer entitled “A Fin is a Limb is a Wing” presents a lucid explanation of how complexity in the plant and animal kingdoms came about, a rebuttal to the argument that organisms such as bacterial flagellae, the eye of a fly, the engineering feat that is the flight feather of a bird, and the human hand and head could only have come into being as a created whole and essentially all at once. Evolutionists agree with Creationists that a limb, a feather, or a flower is a marvel, but Evolutionists do not agree that any or all of them are miracles in the sense of having a supernatural origin.
The genetic underpinnings of natural selection provide a simple and elegant explanation as to how all of those dazzlingly complex structures and beings came to be. Rules govern change: Complex structures evolve through a nearly infinite series of simpler intermediates. Nature is thrifty, modifying old, even ancestral genes, for new uses and even reusing the same gene in new ways, to build something more elaborate. The process of Natural Selection in living creatures has been long and convoluted but because survival was enhanced by some changes, the process was directional—towards greater complexity and enhanced survivability.
Prior to 600 MYA, life was in single cell forms such as Choano- or Cyano-flagellates, tadpole-shaped water sifting sac-like creatures, which still flit about in modern ponds propelled by undulating filaments. About that time, single celled creatures began to give rise to multi-cellular animals. The Choano-flagellates were equipped with the makings of social life and an increase in cellularity and complexity. Proteins made by Choano-flagellates include several essential to maintaining a multi-cellular body which were not and are not directly useful to the single celled animal. Choano-flagellates, the nearest living ancestor to today’s animals have adhesive proteins which lock animal cells together, a feature that may allow the capture of bacteria for food or for metabolic communication between cells, or to sense environmental changes. Minor genetic changes could easily have led to the union of such cells and finally to cohesive and interdependent cellular organisms, including the human organism with its ten trillion cells.
About 400 MYA, Hox genes, which act as master switches to turn on
other sets of genes that guide the formation of distinct regions of animals’ bodies, became active in the developing embryos of then extant animals which resulted in the reshaping of such things as fins into early limbs and rudimentary wings. Hox genes are active along an animal’s body in a particular order, from head to rear, and are clustered along the animal’s chromosomes in exactly the same order. An ancient fish, distantly related to the butterfly fish, gave rise to a creature that could walk on land with an intermediate fish that had primitive fingers on its fins. In the same way, through endless genetic changes a bat’s wing became a retooled hand with a thin membrane over elongated finger bones, resulting from a minor modification in the limb build-ing gene in a mole-like ancestor. Tiny as that change was, the new animal, the bat, could fly.
The question of which came first, the chicken or the egg is answered simply and directly by the Theory of Evolution by Natural Selection. It is the chicken. But, the animal’s complex adult form emerges gradually as the embryo develops. The early embryos of fish, chickens, and humans look very much the same. Genes active in corresponding parts of the embryos of almost all animals activate processes of development down markedly different pathways producing fins, wings or arms. Over the eons of time that the 400-500 million year-long evolutionary processes engendered since Hox genes appeared, small changes in these crucial gene sites resulted not only in the vast array of variety, but also in the remarkable panoply of complexity seen in the plant and animal kingdoms.
A developing fly larva looks as devoid of features as a grain of rice. However, it contains a map, driven by an ancient Bauplan, of the complex creature that it will become. Multiple unseen combinations of genes are active in the tiny organism marking it off into what are early on invisible compartments, each of which has its own shape and function. Some sprout legs, others wings, and still others antennae until finally the invisible but present anatomy becomes visible. One example of the genetic control of this process is the Hox gene called Antp which controls the development of the thorax. In the mouse, a version of that same gene, Hox c6, controls the formation of that animal’s thorax which encloses its heart and lungs. Body proportions can change depending on where particular Hox genes are active. The same Hox gene, Hox c6, switches on at different points along the body. The presence of that gene marks the beginning of the thorax. Its activation results in different species having markedly different neck lengths—short in the mouse, long in the goose and giraffe, and no neck at all in the python.
The genetic combinations that are responsible for laying out the fly’s and mouse’s bodies have nearly identical counterparts in many other animals, ranging from crabs, to earthworms, to lampreys, to Homo sapiens sapiens, a disconcerting finding given the great complexity of the difference between such disparate appearing and functioning organisms. The common ancestor of all of these animals was probably a wormlike creature (Xenoturbella, a tiny Cambrian worm or Corophioides, a large U shaped annelid-worm whose fossils were found in Cambrian Topiates Sandstone in the Grand Canyon of Arizona) that lived about 570 MYA. Those very early bilaterian creatures were endowed with a basic set of body-plan genes which only its descendants used to build their more complex bodies.
The velvet worm, an unexciting little animal, creeps along tropical forest floors on nearly identical pad-shaped legs. It is the closest living relative to the single most diverse group of animals, the arthropods, with their dizzying range of bodily complexity—tarantulas, horseshoe crabs, ticks, and lobsters to name a few. Velvet worms use the same basic set of body-building genes to lay out their anatomy as did Xenoturbella and Corophioides and as do modern arthropods, but with vast changes in complexity over time in which the same genes performed new and different tasks. Changes in body compartments developed into organs for breathing which later developed into wings; and wings, found on multiple segments as seen in fossil insects, were shut off or were used for other building activities. Flies, for example, have just one pair of wings, but a second pair became club-shaped structures called halteres, which help flies remain balanced in flight. Although the segments became different in the different arthropods, the basic machinery for making the appendages is the same throughout present day creatures and those that came before them. Due to genes, especially Hox genes, the great variety in the animal kingdom is largely a variation on a common theme. continued…