“Everything existing in the Universe is the fruit of chance and necessity.”
-Democritus
Evolution is the process of transmission of hereditary results of mutational changes and the eventual establishment of new species. Embryology is the science of biology of animals in the earliest stages of development in utero–the development of an embryo from the fertilization of the ovum to the fetus stage–and the rudimentary plant contained within a plant seed. In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterine wall until the end of the eighth week after conception. Beyond the eighth week after conception–the tenth week of pregnancy)–the developing human is referred to as a fetus. Evo-Devo [Evolutionary Development] is a very new science, a little more than twenty years old, with most of its discoveries coming after the turn of the twenty-first century. Darwin and the scientists who succeeded him were able to demonstrate convincingly that evolution had occurred, and was, in fact, still occurring and what evolution is. The problem in evolutionary studies has been to demonstrate how evolution happened. At the core of its research agenda is the desire to understand the evolution of animal body plans—Bauplans–in terms of changes in embryological developmental patterns and processes.
Embryology studies the process from a single celled fertilized—sperm cell united to an egg cells–to the adult with its millions, billions, or, in the case of Homo sapiens sapiens, some 10 trillion, cells formed into tissues, organs, and body parts, each with a crucial function, coordinated into a functioning and reproducing individual. The evolution pioneers, Darwin and Huxley, knew that development was the key to evolution; but in their time, nothing was known of the actual process. Embryology, heredity, and evolution were intertwined early on, but, lacking fundamental knowledge, the three disciplines became separate and pursued their own lines of interest and evidence. Embryology stalled in a pattern of investigating visible form, ignorant of the underlying mechanisms. The work of Gregor Mendel was not re-discovered until decades after he outlined his laws, and genetics did not begin to prosper until well past the turn of the twentieth century; even then geneticists focused on only a few species. Paleontology focused on the largest time scales of the geologic record, fossil evidence, and the evidence of evolution of the higher taxa. Early on, embryology played no role in what came to be known as the Modern Synthesis. It was not until embryology joined the synthesis with significant–even startling–new information, that it became known how the processes took place.
The initial work by embryologists was the study of the eggs of fruit flies, Drosophila melanogaster. The striking finding coming out of that pioneering work was that most of the genes discovered in the fruit flies which governed their bodily organization were eventually found to have exact counterparts in most animals. The body of work established that the development of vastly different body parts, structure, and functions, long thought to have developed from separate and distinct mutations and developmental pathways, were actually the function of a few ancient adaptable proteins—recall the discussion on Hox genes.
The new discipline of evo-devo relies on the fact that, because early developmental events determine the ground plan for further development, small alterations in the genetic programs underlying early development can lead to drastic changes in phenotype. Scientists developed methodology combining developmental and evolutionary views, which makes sense since evolution occurs as a consequence of changes in developmental mechanisms that produce phenotypic variations which are then exposed to natural selection. The comparison of developmental genes among multiple species grew to be the science of evolutionary development, and this new science bridged the gaps between the other evolutionary study disciplines to produce a synthesized whole that not only explained the fact of evolution, but how it occurs. This combined evo-devo approach has provided unequivocal evidence for the modular nature of embryos—that they have discrete developmental fields, able to change independently, producing safe variation and diversity in evolution.
Vertebrate embryos of any type have common features. The prediction, and the established finding, is that the more common the feature, the earlier it appeared in evolution. The morphological and resulting functional differences that distinguish one vertebrate from another arise later in development, and represent fairly small deviations from the pre-existing plan–the Bauplan. For example, a two chambered heart becomes a three-chambered heart, and then a four-chambered heart over evolutionary time, but the heart itself is universal. Similarly, the vertebrate brain consists of hindbrain, midbrain, and forebrain, but the relative size and organization of each brain division and the functional regions within them can be modified during development according to the evolutionary history of each species. Evo-devo scientists across the world have utilized the tools of genome studies, molecular genetics, and huge computers to expand the world’s knowledge of the veracity of the concept of evolution to the point that it is no longer challenged by valid scientists.
An explosion of information has resulted. The most critical work has been to sequence the entire DNA complement—the genomes—of multiple species, including man and almost all primates. It became evident very early that humans and chimpanzees share nearly 99% of their genes, but even mice and humans have virtually identical sets of some 25,000 genes. The discovery of the ancient genetic “tool-kit” genes is irrefutable evidence of the descent and modification of animals, including humans, from a simple common ancestor. It was discovered that from a few such genes the development of form and function occurred from the turning on and off of genes at different times and places in the courses of embryological development. Evolutionary changes derive from alterations in where and when genes are used, especially those genes that affect the number, shape, and size of a structure.
The genomic system constitutes the developmental program or template. During mitotic divisions, the progeny cells of the zygote become different from one another via the process of cellular differentiation. A higher mammal has enough DNA to encode on the order of two million average size proteins. There are at least 220,000 possible combinatorial patterns of gene expression. Yet, only a tiny fraction of DNA–about 1.5%–codes for the roughly 25,000 proteins in human bodies. About 3%–100 million individual bits—is regulatory. Regulatory DNA contains the instructions for building anatomy; and evolutionary changes within this regulatory DNA lead to the diversity of form, a process which is incremental, additive, heritable, and driven by exposure to environmental stress over time; there have been over 600 million years of animal life and evolution on the earth to foster natural selection.
Fossil forms give evidence of the pervasive use of repeating parts and modular architecture being modified and changed to forge a diversity of animal designs indicative of the activities of those few genes. As described above, the five digit limb design has persisted in multiple forms but with recognizable similarities for 350 million years. The adaptations for compound eyes, dark plumage, fish with antifreeze chemicals and no hemoglobin, color vision, large brains, fire-fly light, zebra stripes, pygmy elephants, fresh water pink dolphins in the Amazon River, giraffes, and men with seven cervical vertebrae, manatees with six, three toed sloths with nine, snakes with dozens, and fish gills, all formed by variations of the same simple fundamental processes. The coelacanth, known in fossil form 350 million years old, also lives today with DNA proteins in the same kinds of genes that humans and spiders share.
Animation of the structure of a section of DNA. The bases lie horizontally between the two spiraling strands. (Photo credit: Wikipedia)
The processes of evolution as identified by evo-devo studies demonstrate the validity of evolutionary concepts. For example, early animals had mouths full of very similar teeth. Later animals developed different kinds of teeth more suited to the needs of their environments. Human teeth include a variety of different kinds of teeth reflective of our retention of the original genes and of the evolution of our complex and successful form. Another characteristic of simple animals is symmetry and polarity controlled by “tool-kit” genes which are in turn regulated by “switch genes”. The same genes—same protein sequences—exist in modern man but are often altered in function, thus representing a remarkable economy in the evolutionary process.
Mutations acted on the standard simple gene functions; and, in turn, the functions altered form over time to result in a different form that was better able to cope with environmental stressors. Limbs separated into upper and lower branches. Upper branches became gills or antennae; lower branches became legs eventually able to lift our ancestors out of the water and to walk on land. Finally, those same limbs resulted in bipedalism producing an animal able to run rapidly to take part in feeding while food was still present and to flee predators. All of these changes required staggering amounts of time—geologic time.
In spite of the rich structural and functional variety of animal body forms found in nature, no more than about 35 different body plans exist, all of which appeared during the Cambrian radiation, over half a billion years ago, indicating an astonishing stability of structure once integrated as a complex form. Although extensive evolutionary changes have occurred since the Cambrian, the underlying body patterns have been highly conserved, a result of the frugal use nature makes in the use of its crucial core genes.
It is crucial to understand the role of homeotic genes. Gene cloning technology was developed in the last third of the twentieth century and gave rise to an extensive understanding of how genes work. Because of its simple four gene structure, the fruit fly was the focus of attention. The work on fruit flies [Drosophila melanogaster] led to the discovery that certain collection of proteins which served as genes were “homeotic genes” which had special effects on particular body regions and parts. The investigating molecular biologists determined that 180 base pair sequences in homeotic genes was a “box” of similarity in otherwise long tracts of DNA sequence. The “boxes” were dubbed “homeoboxes” and the process that resulted from the boxes was called the “homeodomain”. The homeotic genes within these homeoboxes were called Hox genes for short. Laboratories all over the world began a concerted effort to study Hox genes and soon found that altogether similar genes functioned in much the same ways in bugs, earthworms, cows, and humans, a veritable jackpot of information relating to the similarities of genetic function in virtually all extant animals. The animals were full of homeoboxes, or Hox genes.
The stunning finding was that the sequences of amino acids in the so-called Hox genes were almost identical across the wide spectrum of animals. From previous fossil studies, it was known that many of these species had diverged from each other as much as 500 million years ago; and yet, with the advent of evo-devo studies it was realized that they had very, very similar Hox genes. The obvious conclusion was that the Hox genes were so important that they have been preserved for millions of years and hundreds of millions of generations, and that they are the sites of minor changes in amino acid sequence that produce major form changes by increments on a geologic time scale. This was a clear demonstration of how evolution has taken place. Hox genes are the Rosetta Stones of evolution. Disparate animals are made by the same kinds of biological tools, indeed by the same genes.
Close on the heels of the discovery of Hox genes came the discovery, again in the fruit fly, of “tool-kit” genes—a kind of master gene. Other genes were found to act similarly. The Pax-6 genes which govern eye development are found throughout the animal kingdom, in animals with all sorts of eyes. It is highly probable that Pax-6 genes have been present from very ancient times and modified by mutational processes—new tricks for old genes—and are a clear evolutionary link among most species harking back to a very early common ancestor. Distal-less (Dll) genes were found that are responsible for the development of all sorts of structures that protrude from animal bodies—legs, antennae, fins, wings, gills, ampullae, siphons on sea squirts, and tube feet on sea urchins, for example. Similar genes to those that produce tube feet on sea urchins produce tube feet on star fish and the legs of birds. Dll is to the evolution of appendage type forms what Pax-6 is to eyes.
Other such genes determine the development of hearts; others are responsible for production of bony structures; still others produce pheromones. Tool-kit genes were found to be governed by “switch genes”–multiple switches for different subpatterns of function. Again, the frugality of nature is demonstrated; the same genes perform many different functions as determined by the on and off functions of the switch genes (often present as surrounding proteins around Hox and tool-kit gene clusters). The genes first found in fruit flies and their relative frequency of occurrence have counterparts in vertebrates and non-vertebrates, including many much more developed and complex animals such as mammals with their 25,000+ genes. The complexity of animals arises from the many operations taking place at the same time and in succession during development, all of which have been subject to minor incremental mutational changes over the many hundreds of millions of years and billions of generations to produce the animals of today and undoubtedly those remarkable changes yet to come. Subtle changes, often no more than an alteration of position of two or three amino acid pairs, serve to produce the incremental changes of evolution at this molecular biological level, changes that accumulate over time to produce profound differences in form and function which eventually result in speciation, the appearance of a new animal able to procreate with similar members of its population and not with common ancestors. continued…