EVOLUTION, EMBRYOLOGY, AND GENETICS
Evo Devo is short for evolutionary developmental biology- a branch of biology which studies the development of organisms as embryos and then connects it to evolution of animal and species. Darwin’s revolutionary thesis of evolution said that there was a common ancestor to all living species from which diversity evolved over billions of years. He was also aware that embryos of a wide ranging differently looking animals looked remarkably similar in the earlier stages. However, he was clueless about the underlying mechanisms.
Embryology is a study of the embryos to determine the development of an animal from first cell to the completely evolved animal. Genetics is the study of the genes which determine the life processes. Genes are the basic units of life. They are present as discrete units on the chromosomes of each cell. The genes determine the production of proteins, which in turn determines the body organization from a cellular to a gross level. The modern synthesis was a fusion of embryology and genetics. Genetics and its developments became integrated into the explanation of embryology of various animals. Evolution is the study of how diversity arose from common ancestors and species. The study of evolution included palaeontology, which is the science of fossil animals and plants.
The Third Synthesis is the bringing together of evolution, embryological development, and genetics into a common field- Evo Devo. This exciting branch now tries to integrate evolution with embryology at macroscopic level and genetics at a microscopic level. This book by Sean Carroll gives a wonderful overview of this rapidly developing field.
GENOME, DNA, GENES AND PROTEINS
A genome is an organism’s complete set of deoxyribonucleic acid (DNA), a chemical compound that contains the genetic instructions needed to develop and direct the activities of every organism. DNA molecules have two twisting, paired strands. Four chemical units called nucleotide bases make each strand. The bases are adenine (A), thymine (T), guanine (G) and cytosine (C). Bases on opposite strands pair specifically; an A always pairs with a T, and a C always with a G. The human genome holds 3 billion of these base pairs, which reside in the 23 pairs of chromosomes within the nucleus of all our cells.
A gene is a part of the DNA molecule consisting of a certain number of base pairs ranging from as low as 1000 to even 2 million. On average, human genes are 27,000 base pairs long. What exactly does a gene do? The traditional mechanism for gene action was by its transmission of information into RNA, which in turn produces a specific protein or enzyme. The definition of a gene was hence a functional unit producing a specific protein (through an intermediary RNA). The gene product has in turn a specific role in the form and function of the organism. However, with increased scientific discoveries of genes having a role in regulating other genes, the scope of gene definition has now extended.
A broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affect an organism’s traits by a functional product or by regulation of gene expression.
ANIMAL ARCHITECTURE AND COMMON THEMES
Animals evolved from a single ancestor about 600 million years back and have diversified into an amazing number of species from a small bacterium to dinosaurs. Many have become extinct during the path of evolution; in fact, 99% of all animals evolved have gone extinct. The author says despite the amazing diversity of animal architecture, there is an underlying common plan of design.
Modularity, symmetry, and polarity are the near universal features of animal design, and certainly of larger, more complex animals. Similar body parts make animals amazingly related to each other. Individual animals have the same kind of parts, like building blocks of a Lego type. A repeating modular architecture of the vertebrate body plan has been constant over millions of years of animal evolution. Fossils show a pervasive use of repeating parts and modular architecture in the creation of animal designs. Despite a similar modular construction of vertebrates and other animals; variations in their number and kinds accounts for the tremendous variation of animal design.
As an example, frogs have about 12 vertebrae, humans have 33, and snakes have a few hundred of them. Though the number vary, the plan stays the same. Similarly, the basic five-digit design of limbs has persisted for more than 350 million years, although some variations have occurred in their number (camels have two, rhinos have three).
The similar organs which have derived from the same ancestor are called as ‘homologs’. The forelimbs of salamanders, mice, and our arms are homologs. Changes in the number and kind of these homologs are the principal theme of evolution. Willinston’s law states that, ‘It is a law of evolution that the parts of an organism tend toward reduction in number, with the fewer parts greatly specialized in function’. Wings in birds, for example, are modified limbs.
Symmetry and polarity are two other important components of animal design. Most animals are bilaterally symmetrical around a central axis running on the long axis of the body. They have hence matching and mirroring right and left sides. Similarity polarity has three axes: ‘head to tail’, ‘top to bottom’ (back and front in us as we stand up), and ‘near to far’ from the body (with respect to the organs projecting from the body like the limbs). The example of the hand clarifies the concept of polarity. It has three axes oriented by the thumb to the little finger, back to palm, and wrist to fingertip directions.
So, where do these instructions to determine modularity, symmetry, and polarity come from? The answer is-DNA.
MONSTERS, MUTANTS, AND MASTER GENES
There have been many scientists who have devoted their entire career spans to the study of mutants and the so-called monsters- animals with extra organs and at wrong places. Spemann was a renowned embryologist who realized that there were critical organizer areas in the embryo which direct the development of the total body.
He discovered that in tadpoles, a specific region of the initial mass of dividing cells called the dorsal lip of the blastopore, organized the top or the dorsal part of the embryo into neural structures. This in turn initiates the development of another embryonic axis. The Spemann organizer revealed that one of the ways of order in development is by interactions between one part of the embryo and other parts. These interacting influences works across many scales; from the gross level to microscopic. Studies on limb development and polarity of digits have clearly shown that there is a well-placed system that cues cells to what they are going to finally become. Initially, the embryo is a mass of dividing cells. The great mystery was how the cells decide to migrate and differentiate into the various specialised organs, tissues, and functioning cells.
All organizers share the property of influencing the formation of pattern, or morphogenesis, in tissues or cells. This they do by producing special substances dubbed as morphogens. A major difficulty was in identifying the specific morphogens in the huge chemical soup of molecules formed by the organizer cells.
The study of ‘homeotic mutants’, that is, mutants with one body part converted to another body part (like the antenna into a leg or the forelimb into a hindlimb), finally showed ‘master genes’ at work. These master genes were critical in determining the organization of the embryonic cells at various levels to form the final body.
TOOLKIT GENES AND COMMON RECIPES
The discovery of toolkit genes is one of the most exciting chapters in the history of genetic science. This could explain the paradox of mice and humans having nearly identical 25,000 genes or chimps and humans having 99% identical DNA, and yet we are different in obvious ways.
Genes are used in a specific manner at a specific time at a specific place. Though each cell has the DNA to build the entire body, in any given cell specific functions are taking place. The liver cell is producing enzymes for digestion, the islet cells are producing pancreas, the heart cell is working like a mechanical contraption, and the brain cell is co-coordinating the body. How does this happen?
The answer is the toolkit gene complex. These are master genes which are activated at specific times and are responsible for activation of the actual genes and their products. There are now discovered many such master genes in the tool kit whose products act at distinct levels of DNA activation and expression. These toolkit genes are hence development genes that control embryonic change. They are expressed depending on the location and signal position once the axes have formed. The astounding news was that these toolkit genes or the homeobox genes were exactly the same in many animals across many orders of scale. These are called Hox genes for short. The Hox genes are so important that that their sequences have been preserved through almost 500 million years of evolution!
The depth of similarity between different animals extended not just to the sequence of the genes, but to their organization in clusters, and how they are used in the embryos, says author Sean Carroll. Clusters of Hox genes shape the development of animals as different flies and mice, and includes practically all animals, including humans. Disparate animals were using the same genes and the same kinds of tools.
Similar genes like Pax-6 genes are used in the formation of the eye in the entire animal kingdom. Distal-less genes or Dll genes are used for modification of the limb appendages. The super-controller of development, Hox genes tell bodies where to develop heads, tails, arms, legs, in every major animal group. Hox genes are in mice, worms, humans, and they are inherited from a common ancestor.
Toolkit genes are old, present in all animals and they do nearly the same thing in all animals. Hence, conserved genomic material forms an important part of the molecular building blocks of life.
Evo-Devo hence proposes that evolution uses the same ingredients in all organisms, but tinkers with the recipe. By expressing genes at separate times in development and/or in various parts of the body, the same genes can be used in different combinations to allow evolution, phenotypic diversity, and innovation. Animals look different not because the molecular machinery is different, but because various parts of the machinery are activated to differing degrees, at different times, in different places and in different combinations. The number of combinations is huge, and so this is a plausible explanation for the development of complex and diverse phenotypes from even a small number of genes.
For example, humans have a mere 21,000 to 25000 genes in our genome, and yet we are one of the most complex products of evolution. This is now the prevailing theory: all animals are built from essentially the same set of regulatory genes—a genetic toolkit, and that phenotypic variation within and between species arises simply by using shared genes differently.
A conserved genome can generate novelties through rearrangements (within or between genes), changes in regulation or genome duplication events. The combinatorial power of even a limited genetic toolkit gives it enormous potential to evolve novelty from old machinery.
Over the last decade or so, interest in the toolkit gene phenomenon has spread to the study of animal behavior too. Though behaviors are complex and regulated by many genes, behavioral ecologists have long observed similarities in behavioral phenotypes across diverse species, presumably the result of shared ecological conditions and selection pressures.
A few key discoveries in behavioral genetics provided the first evidence that there may indeed be genetic toolkits for behaviour. One famous example involves the genetic regulation of foraging behavior. In vertebrates including humans, FoxP2 and similar genes have been repeatedly associated with speech, song, and other types of vocalizations, a second example of a behavioral toolkit gene. These findings and the potential significance of the widespread occurrence of behavioral genetic toolkits has stimulated added research in this field of animal behaviour.
MAKING BABIES
Development of an animal from the first fertilised cell is an amazing drama. Some animals have a rush job like frogs and flies, whereas human development is at a gentle pace. The important questions which arise while seeing an embryo is how does it know which is the head and which is the tail? Or how are the top and bottoms decided? How does the organization of cells at specific points in the embryo take place so that they specialize into tissues and organs? The cells are all the same at all places in the body. However, activation of a few genes allows the cell to become the eye, skin, bone, or the liver. At what point of the embryo’s development is the cell’s fate sealed?
Fate mapping experiments have given many elegant atlases on the geography and development of the embryo. Early in the development of the embryo, different axes develop. Longitudes, latitudes, altitudes, and depth define clearly. The cells at specific coordinates become specific tissues and organs in a very organised manner. Just as a coordinate on the map of the globe precisely defines a place and a country.
The geometry of the embryo’s coordinates, with its parallel and intersecting lines of longitude and latitude, imposes some spatial order on how the program of the tool-kit gene unfolds. Tool kit gene expression takes various geometrical forms like bands, stripes, lines, spots, dots, or curves which finally determines the animal shape and size. The tool kit’s gene logic determines in organizing, subdividing, sculpting, and specifying parts of the embryo.
All vertebrate embryos pass through a stage where they look remarkably similar, whether of an elephant or human. Apart from the head to tail axis, there is a well-defined north-south axis where the different tissue layers form. There is a stiff rod of cells on the back called the notochord and surrounding it, regular, paired bumps of somites form a pattern of repeated modules along the entire length of the animal. This modular pattern has some important implications in the growth and development of various organs and tissues.
The complexity of an animal arises from the parallel and sequential action of tool kit genes-dozens of genes acting at the same time and place, many more genes acting at various places at the same time, and hundred of tool kit genes acting in sequence as development progresses. Hence, a symphony exists where one tool kit may activate at many places or many tool kits at one place leading to the development of the complex animal.
THE DARK MATTER OF THE GENOME: OPERATING INSTRUCTIONS FOR THE TOOL KIT
Only 3% of our DNA is a gene sequence for a specific protein or a function. There is an amazing 97% of DNA which do not seem to have any function and is traditionally termed ‘junk DNA’. This is not true. This is akin to the dark matter playing a silent yet powerful role in shaping the universe.
Elegant experiments show now that instructions for the activation or silencing of the tool kit genes embed in this ‘dark matter’ of the genome as genetic switches. These switches surprisingly and crucially control exceptionally minute details of individual tool kit action and anatomy. These switches are the key actors in development and evolution of animals. About 2-3% of the dark matter are genetic switches that control how genes make the animal. Hence, even if protein making genes are similar in number in mice, worms, and humans; even if the tool kit genes working on these genes are remarkably similar for millions of years; the symphony of the genetic genes acting at various places and various times make the diversity and complexity of the animal.
An example is of lactase produced by bacteria to breakdown lactose. When lactose is absent in the environment, a lac suppressor gene switch falls on the DNA producing lactase to inhibit its work. When lactose is present, the lac suppressor gene falls off the DNA sequence allowing it to express the production of lactase enzyme. This is how genetic switches work.
The genetic switches work as GPS devices getting a positional fix by integrating multiple inputs. The switches integrate positional information in the embryo with respect to longitude, latitude, altitude, and depth, and then dictate the places where genes turn off and on. Tens and thousands of these switches make way for an overwhelming number achieved by combinations and permutations. Finally, the beginning of spatial information in the embryo traces back to the asymmetrically distributed molecules distributed molecules deposited in the egg during its production in the ovary that start the formation of the two main axes of the embryo. The egg indeed before the chicken, says the author. Amazingly, asymmetrical distribution of matter in the early moments of big-bang gave the universe its matter distribution.
An average size switch is several hundred base pairs long on average. Within this span there are anywhere between 6 to 20 signature sequences for different proteins. The response of a switch to a longitude, latitude, altitude, or depth input depends on the presence, number, and local arrangement of signature sequences bound by tool kit proteins. There is deployment of tool kit proteins along any of the axes or within any specific tissue. Tool kit genes are used and reused repeatedly in development in different contexts to shape the growing embryo. At least 10 switches control each tool kit gene. Because the combination of inputs determines the output of the switch, and the potential combinations of inputs increase exponentially with each additional input, the potential output is virtually endless.
Finally, the developmental steps executed by individual switches and proteins connect to those of other genes and proteins. Larger sets of interconnected switches and proteins form local circuits that are part of still larger networks governing the development of complex structures. Animal architecture is finally the product of genetic regulatory network architecture.
From development to evolution is another leap for the role of these genetic switches. It is amazing that switches enable the use of the same tool kit genes differently in different animals. Because individual switches are independent information-processing units, evolutionary changes in one switch of a tool kit gene or in a switch controlled by a tool kit protein can alter the development of one structure or pattern without altering other structures. This is the key of evolution of modular bodies and body parts-how, we develop an opposable thumb, and flies evolve a hindwing.
PALAEONTOLOGY, ANIMAL EVOLUTION, BIG BANG OF CAMBRIAN EXPLOSION
Palaeontology is the study of fossils to make sense of the evolutionary drama. The Earth formed 4.5 billion years ago. Though the primitive forms of life might have kicked off between 3 and 4 billion years, but for 3 billion years it was small and simple organisms (several millimeters or smaller). 525 to 550 million years ago, there was a sudden and rapid geological appearance of complex forms around the world. This is the Cambrian Explosion-the Big Bang of animal evolution. What might have been the triggers? In the Precambrian phase, 600 to 570 million years ago, the size and shape expanded to animals a few centimetres in size, termed as the Ediacaran fauna.
Embryological studies in Evo Devo has enabled to figure out the role of genes in the ignition and expansion of Cambrian Explosion. The stunning message is that all the genes for building large, complex bodies long predated the appearance of those bodies in the Cambrian Explosion, by at least 50 million years! The main story of many groups of animals is that of evolving different numbers and kinds of repeated body parts.
Living organisms divide into two main groups- the Prokaryotes and the Eukaryotes. Complex multicellular organisms, including animals fall into the latter group. Insects and vertebrates are representative of the two main different branches of the animal tree- protostomes and deuterostomes, respectively. We are the vertebrates. Insects and vertebrates appeared first in the Cambrian Explosion period. Fossil record stops at this. We do not have any fossil evidence of the common ancestor termed the Urbilateria from which the protostomes and the deuterostomes split off.
Evo Devo comes to help here. The logic of Evo Devo is that we can make inferences based on what the descendants share. The basic premise is that whatever is common to two or more groups is likely to have existed in their last common ancestor. By applying this logic to the knowledge of development and genes, scientists have made inferences about the genetic make-up of the common ancestors. Evo Devo tells that the last common ancestor of protostomes and deuterostomes was bilaterally symmetrical. It also tells that Urbilateria had a tool kit of at least six or seven Hox genes, Pax-6, Distal-less, tinman, and a few hundred more body-building genes. It also had some kind of eyespot or light sensing organ made up of photo-sensitive cells arranged with some geometry. It may have had projecting structures-the forerunner of limbs and fins.
Evo Devo has shown that an ancestor existed equipped with all the necessary genes for building complex bodies and possessing some initial level of anatomical complexity. Arthropod (or insects) evolution is a story of increasing segment and limb type diversity in the Cambrian period. This is in turn a product of generating a greater number of unique zones in the embryo in which specific individual or combinations of Hox genes get an expression. This relative shifting of Hox genes is thus one of the mechanisms underpinning Williston’s law-the specialization of repetitive parts requires that the various parts fall into different Hox zones.
Significantly, even evolution of higher vertebrates is not due to more genes but due to changing embryo geography by shifting the zones of Hox genes, though in larger numbers. It is surprising that four Hox clusters has been stable throughout the evolution of amphibians, reptiles, birds, and mammals. Frogs, snakes, dinosaurs, ostriches, giraffes, whales have evolved around a similar set of four Hox gene clusters!
Hence, significantly, evolution of the two of the most successful of animal groups-arthropods and vertebrates- have shaped by similar mechanisms of shifting Hox genes up and down the main body axis. The individual groups of animals are hence not unique but are variations of a common theme based on the tool kit genes and their activation symphony. As a step back in the scheme of things, switches of Hox genes play a role in this selective activation. Evolutionary changes in the ‘Switch genes’ leads to evolutionary changes of the Hox genes. Changing the sequence of the switches allows for changes in the embryo geography without disrupting the functional integrity of a tool kit protein. What causes the Switch genes to change and evolve? Watch the space.
Evo Devo has hence demonstrated three essential elements of early animal history. First, the last common ancestor of the two main animal tree branches was a genetically and anatomically complex animal despite an absent fossil evidence. Second, the full genetic tool kit for body-building was in place, but its potential untapped for a long time. And third, the potential of tool kit genes realizes through the evolution of switches and gene networks and the shifting of Hox genes starting with the Cambrian period and continuing in present times.
And what finally drove the Cambrian explosion? It was an ecological phenomenon, says the author. The evolution of larger and complex animals paved the way for still larger and complex animals. As the Big Bang unfolded, the pressure of ecological interactions and competition among increasingly diverse animal species drove the evolution of higher and more complex structures-eyes for vision, hearts for circulation, and appendages for walking, swimming, and grabbing. Hence, genes in the tool kit are important actors, but they represent only possibilities, not destiny. Ecology on global scale drove the drama of Cambrian Explosion.
LITTLE BANGS AND EVOLUTIONARY INNOVATION TO BIODIVERSITY
The main secret of evolutionary innovation to new forms is to work with what is already present, says the author. Spinnerets in spiders or wings in vertebrates did not arise de novo on the body from the top or sides of a four-limbed animal but were as modifications of existing limbs. Nature is more of a tinkerer working and cobbling together available materials, constantly modifying, constantly retouching structures over millions of years. It does not work as an engineer would with a preconceived plan and specialized tools. This extends to the gene level in the sense that same old genes go on diverse paths. The evolutionary innovation works by a zigzagging path from A to B, not to B from scratch.
Multifunctionality and redundancy are evolutionary tricks recognized by Darwin too. Any part of a multifunctional structure that is partly redundant in function sets the stage for specialization through the division of labor into two structures.
Modular architecture consisting of repeated body parts in vertebrates and arthropods is the most important reason for the evolutionary success and biodiversity. This has led to different adaptations that co-occur on the same animal allowing for modification and specialization of individual body parts, sometimes in the extreme, independent of other body parts. Modular design modification is responsible for the evolution of hundreds of vertebrae in snakes and many long fingers in bats upon which to extend a wing membrane.
Underlying an anatomical modularity in adults is a modular embryo geography and a modular genetic logic of switches. These switches allow evolutionary change to occur in one part of the body independent of other structures, Switches are the secret to modularity, which in turn is the key to evolutionary success of biodiversity.
Innovation allows for invasion of new niches, and invasion leads to more diversity. Life forms started 3.5 billion years ago, but for most of 3 billion years it was in water. These kinds of evolutionary innovations finally led to the arrival on land of animal forms. Coupled with other evolutionary adaptations to high oxygen levels and a symbiotic relationship with the plants, there was a rapid expansion of biodiversity; however, the basic designs remained remarkably same across various species at the level of embryo and the genes.
BUTTERFLY SPOTS
The wings of the butterfly are a modification of the limbs. The author takes us on an amazing journey of the evolution of thousands of spot patterns on the wings of the butterfly. The wing spots is a terrific example of ‘mimicry’ in the natural world. The spots of a butterfly species may evolve by natural selection into spots of another species to avoid eaten up by predators. Tasty butterflies mimic the wing spots of noxious butterflies to escape being eaten. How do they do that? Tool kit genes, of course-again!
The distal-less gene is a tool kit gene which is responsible for the evolution of the thousands of patterns of wing spots. The selective excitation and inhibition of the same set of genes by switches at different times and different places in the embryonic development of the butterfly gives the spots to its wings. And natural selection favours those which are not preyed upon.
BLACK AND WHITE
There is a fascinating chapter on the black coloration of animals and its implications in evolutionary survival. Stephen Jay Gould asked, ‘Is the zebra a white animal with black stripes or a black animal with white stripes?’ The author settles for the black animal/ white animal verdict.
Coloration of animals plays a critical role in their interaction with other species and with the own species. The natural history of coloration holds an important place in evolutionary biology, particularly as examples of natural and sexual selection.
Melanism is a condition where a species displays greater areas of dark coloration in place of other colours. Dark colouration helps moths, for example, to escape bird predators when soot of industrial areas blackens the trees.
Melanin is a composite pigment of two sub-types which gives the black colour to the skin. Eumelanin and phaeomelanin, responsible for the black-brown and reddish orange coloration of fur or the skin of the animal. The melanin production is an interplay of many proteins and genes. An important protein is called the MCIR or the melanocortin-1 receptor. MCIR sits on the cell membrane with one half outside the cell and one half inside. When a hormone called MSH binds to MICR, a chain of reactions inside the cell leads to production of eumelanin and hence black colouration. Similarly, a protein called Agouti blocks the MICR receptor and leads to production of phaeomelanin. Pigment type hence depends on the state of activity of the MICR protein.
The MCIR and the Agouti genes controlling the MCIR protein and its inhibitor Agouti respectively is capable of many mutations which leads to selective expression and inhibition of melanin production. There are other genes involved too, but the idea is that there are few genes controlling the melanism in animals. However, the switches controlling these genes in turn play the most crucial role in pigmentation as we have seen already. The selective play of gene switches leads to selective expression of genes and as a consequence the presence or absence of melanin pigmentation. It is again mutational changes in the switches holding a key to the diversity of melanic pigmentation in the animal kingdom. The black colours, stripes, and spots are amazingly decided when the embryo itself is forming.
SELECTION, GENES AND FITNESS: HOW MUCH OF AN ADVANTAGE MATTERS
Population genetics is a branch of genetics which deals with the variation among individuals, the genetic basis for it, and the changes in the frequency of forms and genes in evolution. The author briefly talks about it. An important formula for determining the time, in generations, for a mutation to spread throughout the population is:
Time=2/s In(2N)
Here N= the number of individuals in the population and In is the natural logarithm. The s refers to ‘selection coefficient’ which is a measure of the relative fitness of the new mutation in terms of survival and sexual fecundity.
For example, if a mutation leaves 101 offspring as compared to 100 of the non-mutational species, the s becomes 1% or 0.01.
As an example, if s is just 0.01, N is 10,000, then the time in generations would be 1980 for the mutational species to completely establish its presence. For a moth it would mean 2000 years only. If s=0.001 meaning a 0.1% advantage, the mutation would be fixed in 20,000 generations.
The powerful idea is that even with an exceedingly small advantage, a mutation spreads in a geologically brief period of time. A selection coefficient of 0.2 to 0.5 is exceedingly high. As seen in some moths, such coefficients allow for huge selective advantages. Similar calculations also show that mutations that cause even a slight disadvantage have a very slim chance of spreading throughout the population and in fact, get eliminated in the scheme of natural selection.
THE MAKING OF HOMO SAPIENS
Charles Darwin said famously that the difference in the mind of animals and humans is of degree and not of kind. The stand has not changed with advances in science and genetics. What makes us different?
Emerson Pugh says, ‘If the human brain was so simple that we could understand it, we would be so simple that that we couldn’t’. 6 million years ago, humans separated from our closest relatives in the animal kingdom-the chimpanzee, with whom we share we share 99% of genetic make-up.
Hominids refer to both humans and apes of African origin; hominins refer to only humans and our ancestors till the time we split from the common ancestor 6 million years back. We belong to the Homo sapiens line of the hominins which includes between 15 to 20 discovered species including Homo neanderthalenis, our closest hominin relative. We have been around for about 200,000 years only in this large canvas of time. The morphological and developmental characteristics of H. sapiens are: larger body size, larger brains, longer legs relative to torso, and smaller teeth. Bipedal locomotion, upright skull on vertebral column, reduced body hair, s-shaped spine, pelvic dimensions set the hominins apart from the chimps.
Our species is the latest in the chain of hominin species to have evolved. We are neither the last (that is, if we do not completely blow ourselves up completely) nor the best. There will be more to come. We may have eliminated our nearest hominin relatives, the Neanderthals, by sharper intelligence or may have even interbred with them. Interestingly Neanderthals had larger body and brain sizes than Homo sapiens. Maybe, they were eliminated because they were more peaceful! The author says firmly that there was no interbreeding. Both co-existed after splitting from the same ancestor 500,000 years ago. 300,000 years ago, Neanderthals died leaving us alone as the most intelligent left on the planet. The fact was that Neanderthals also used tools, made fire, and had other signs of culture, language, and self-awareness.
The relative increase in brain size compared with body mass is a distinguishing feature of humans. Absolute brain size is not necessarily an indicator of greater power. Whales and elephants have larger brains than humans, but as a percentage of body weight, which makes the difference, human brain is 15-20 times larger than those animals. The brain is an expensive energy consumer, drawing up to 25% of an adult’s energy and 60% in infants. Why did our brain sizes increase? The author claims that one of the strongest reasons could be adaptation to climactic change as the planet got colder. The effects of climate change on food availability, water, hunting, and migration may have selected for hominins better adapted to constantly changing conditions.
The challenge is to find out what exactly in the brain accounts for the differences in human capabilities and puts him apart from all the others. The major paradox is that humans share 98.8% of DNA bases with chimpanzees and 99% of human genes have a mouse counterpart. 96% of mouse genes are exactly in the same order as the human genes. Our protein coding genes are only 20000 to 30000 in number, which is the same of many animals including the roundworms. So where is all the complexity coming from? Our most critical areas in the brain are the Broca’s area and the Wernicke’s area responsible for speech and language. Speech and language have played the most vital role in the evolutionary domination of humans over all the other species. The author says that it is not the number of genes or the proteins they encode which account for the complexity of humans, but because of changes in the control of genes.
Genetic and anatomical evidence suggest that inactivation of a certain protein called MHY16 is associated with reduction of temporalis or the jaw muscle in hominids. Reduction in jaw musculature, and the forces imposed on the mandible, would reduce the stress on the bones in the skull. This allowed the braincase to become thinner and larger allowing the brain to grow bigger. Similarly, mutations in FOXP2 gene or the genes controlling them in the Homo sapiens could have allowed speech to flourish in humans. These are only a few pieces of the puzzle of our defining traits-bipedalism, skeletal form, craniofacial form, brain size, or speech; however, they may not be the only ones or the most important.
Hominin evolution is the result of selection for variants of many genes, responsible for small increments in differences in size, shape, and tissue composition, over sustained intervals of many thousands of generations, says the author. To add the complexity, human brains are different in its ‘microanatomical structure’, which include the interconnections between cortical regions, the architecture of the local wiring circuits, or the arrangement of neurons in the cortex. This could be the defining reasons of our domination today. What accounts for these changes is a part of ongoing intense research.
ENDLESS FORMS MOST BEAUTIFUL- CONCLUDING REMARKS AND SUMMARY
The final chapter is a recap on the importance of Evo Devo in a continuing story of evolutionary synthesis. The integration of embryology with molecular genetics and paleontology has led to key discoveries and has settled some unresolved questions with quality evidence. Evo Devo provides a new method of teaching evolutionary principles in a more effective framework. It has clarified the deep principles involving development and genes underlying the unity and diversity of life. The evolutionary principles are illustrated in a more tangible way by this field and that plays a critical role in the teaching of evolution to society, says the author. And finally, a broader understanding of the human impacts on evolution has ramifications in the fate of the endless forms of Nature, including humans.
Evo Devo is a cornerstone of a more modern synthesis. The key principles of evolution are explained by Evo Devo. On descent with modification, Evo Devo shows that the tools for making the animal kingdom are ancient. The large-scale trends in animal design and evolution have a common basis; and are enabled in the properties of various switches embedded in the non-coding area of the genome-the ‘dark matter’ of the genome. This leads to the tremendous complexity and diversity of living matter. Novelty and innovation probably do not arise from new genes but due to selective activation and deactivation of existing genes and structures. Think limbs, fins, and wings having a common ancestor.
Evo Devo has shown importantly that a handful of conserved tool kit genes for over 500 million years is responsible for changes at the macro and the micro level. It shows that what is true of the species in terms of individual mutations is true for the kingdom too. The small scale and the large-scale changes have the same underlying mechanism, and this is the most compelling message of Evo Devo.
TEACHING OF EVOLUTION
A question was asked in a survey of 21 countries to review the public understanding of evolution: ‘Human beings developed from an earlier species of animals. In your opinion, how true is this using a four-point scale-1 definitely true, 2 probably true, 3 probably not true, and 4 definitely not true?’
The best country was East Germany with a mean score of 1.86. Great Britain had a mean score of 2.18, Canada at 2.45, and surprisingly the United States stood at the bottom of the table with a mean score of 3.22! The author notes ruefully that this perhaps a good thing because US now can only move upwards in the score.
The National Science Board in 1996 took another survey. To a statement, ‘The earliest humans lived at the same time as the dinosaurs’ requiring a yes/no answer, 32% said yes and 20% did not know (an amazing 52% of the total people polled). The author is highly disturbed and pained at the public ignorance of one of the most established tenets of scientific discipline in the wealthiest, most powerful, and technologically driven nation. He terms this scandal of ignorance at par with not knowing the principles of Constitution. He feels strongly that Evo Devo teaching can reverse this ignorance to some extent.
The visual nature of Evo Devo perspective along with the linkage of genetics to fossil studies may greatly improve the public understanding of evolution, the author says. However, right from the beginning the textbooks must insist that evolution is more than just a topic in biology-it is the foundation of the entire discipline.
This lack of focus has allowed creationists and ‘intelligent design’ groups to interfere with the teaching of biology and making it faulty. The author quotes Goethe who said, ‘Nothing is worse than active ignorance’, and the author is worried at the agenda of the ‘lost souls’ to thwart science and education. The Church is finally taking positions of acceptance much to the relief of scientists, but the author cautions that science and evolution is best promoted by scientific knowledge and education and not by attacking religion. Similarly, religion would do better to promote its theologies and teachings rather than to attack scientific views. Classically, religion looks at the gaps in the understanding of science and promotes them as evidence of god-the ‘god of gaps’. However, as the gaps in understanding close down by scientific method, the religious fundamentalists have less space to wiggle.
The author is firm in telling that nothing of intelligent design should ever be in the teaching curriculum despite any attraction of allowing disparate opinions and the dubious writings of some mainstream scientists- the author mentions Michael Behe here- for creationists to raise their heads. Science and religion should stay within their limits and not get into each other. Unfortunately, western authors do not consider Hinduism and Advaita in equations of science with religion. In 1890s, Swami Vivekananda was amused at the resistance to evolution in the western world. He simply said evolution is a fact of nature and Advaita would never have problems with it. The sad part is that there are many religious denominations which does not have any problems with scientific method, especially evolution. But, that is a topic for a larger debate.
THE FINAL STAKES
Humans numbered 10 million before agriculture started. It reached 300 million by CE 1. The population reached 1 billion in 1800. We are around 6-7 billion and are expected to reach 9 billion in the next fifty years. In 10,000 years, the humans have increased a thousand-fold. In geological time-scales, this is not even the slightest blink of the eye, yet we have profoundly impacted life of all Earth. Animal species are getting extinct at an alarming rate and perhaps in the phase of the sixth mass extinction. The last was 65 million years ago which wiped out dinosaurs and allowed humans to come forward. However, it was caused by meteorites and natural disaster. This extinction is solely because of human activity and the persistent drive for mastering over Nature. Humans have this belief, scientific and otherwise, that symbiosis and harmony with nature is not required; and the only need and purpose of the rest of Earth and its species is to serve us. There have been cautioning voices, but they have been totally silenced. Such an attitude does not augur well for the long-term survival of the planet and its endless beautiful species. Finally, we may go down with everyone as a consequence of our actions.
Understanding Evo Devo and appreciating the diversity and richness of the living kingdom may help us in protecting and cherishing Nature. On these final words of optimism, the author ends one of the most fascinating reads of my life. It is a must recommend to every English knowing person on Earth. Schools could consider this book as an essential reading and it would also be a recommendation to translate the book in as many languages as possible
RANDOM MUSINGS 