A zygote is a diploid cell that fuses haploid gametes into a fertilized ovum. The question for our purpose is how the fertilized egg develops from a single cell into a living organism.
As castles are made of brick and stone, organisms are constructed by cells. The animal can be unicellular (one cell) or multicellular (many cells.) In addition to the two types of cells, there are two types of organisms. There are the ancient prokaryotes and the (relatively) new eukaryotic organisms. I’d like to take a moment to note the differences between the two fundamental cells of living systems. I’ll start where nature started–with the prokaryotes.
Prokaryotic cells are not as complex as eukaryotic cells. They have no stable nucleus and the DNA is not contained within a membrane or separated from the rest of the cell. It is confined to a region of the cytoplasm called the nucleoid. Using bacteria as an example, I will attempt to describe the anatomy of the prokaryotic cell:
The obvious starting point is the capsule, that, when found in some bacterial cells, acts as an additional layer that protects the cell from other organisms. The capsule also helps the cell bind to surfaces and take in nutrients. The cell wall consists of a covering of cells that protect the bacteria and give it shape. The plasma membrane surrounds the cytoplasm and the cell and allows for substances to pass into and out of the cell. The Pili, a rare word to come across for sure, is a hair-like structure on the surface of the cell. It attaches to other bacterial cells. The short pili (fimbriae) allows bacteria to attach to surfaces and spread.
The flagella, the creationist’s wet dream, has a long protrusion that affords cellular movement. The ribosomes are cells responsible for metabolizing proteins. The plasmids are circular structures of DNA, although they are not involved in reproduction. Finally there is the nucleoid region, where the cytoplasm contains a single DNA (and bacterial) molecule. Prokaryotes also reproduce asexually by means of binary fission.
Binary fission begins with the single DNA molecule replicating both copies attaching to the cell membrane. Then, the cell membrane begins to grow between two DNA molecules. The bacterium, once it has doubled in size, the cell membrane begins to pinch inward. A cell wall forms between the two DNA molecules dividing the original cell into identical daughter cells.
The most notable feature that differentiates these complex cells from prokaryotes is the presence of a nucleus, a double membrane-bound control center separating the genetic ‘material,’ DNA (Deoxyribonucleic Acid), from the rest of the cell. In addition to the plasma membrane, eukaryotic cells contain internal membrane-bound structures called organelles. Organelles, such as mitochondria and chloroplasts, are both believed to have evolved from prokaryotes that began living symbiotically (we’ll discuss symbiosis itself later on) within eukaryotic cells. These organelles are involved in metabolism and energy conversion within the cell. Other cellular organelles within eukaryotic cell structure carry out the many additional functions required for the cell to survive and reproduce. Eukaryotic cells can reproduce in one of two ways: meiosis (sexual reproduction) and mitosis (cell division in producing identical daughter cells as within asexual species.)
Both eukaryotic and prokaryotic cells have a plasma membrane, a membrane that surrounds the cell. However, only eukaryotic cells have an endomembrane system, a collection of intracellular membrane-bound organelles (such as vesicles, lysosomes, endoplasmic reticlum, a golgi apparatus, and mitochondria.
This system of organelles functions to transport material into and out of the cells; everything inside the plasma membrane is floating around in cytoplasm. I feel that it would be proper to treat the eukaryotic cell, and define its functions, as thoroughly as possible. All of the animals and human beings are eukaryotes. Men, kittens, and dandelion puffs.
Eukaryotic cells are so called because they house a nucleus. This nucleus houses genes (DNA) and is contained within a membrane. This separates it from other cellular structures. As I’ve mentioned, prokaryotes have no true nucleus. In the prokaryotic cell, the DNA is not separated from the rest of the cell; it is bound within the nucleoid.
Eukaryotes grow and reproduce in a quite different manner than the prokaryotes; they reproduce through a process called mitosis. Reproducing sexually causes cell division. This process is called meiosis. Despite their differences, both types get their energy to grow, and maintain, normal cellular function by means of cellular restoration.
Cellular respiration has three distinct stages: glycolysis, citric acid cyde, and electron transport. Most cellular respiration takes place with mitochondria, another organelle. In prokaryotes this happens in the cytoplasm within the cell membrane. This brings us to the organism.
Biological organisms can be explained in four ways: morphological structure, function, chemistry, and biochemistry. Chemistry, in living systems, shows us what they’re made of. The scientists of our time know a lot about this type of chemistry—it is a chemical constituent on which living systems depend. There are, as well, chemical species: when an organism takes in nutrients, the organism is taking, from the smallest of cells, thousands of chemical species.
Biochemistry defines a set of chemical reactions in living systems. This allows organisms to synthesize a large number of chemicals. Structure is the phylogeny of the organism, what we see with our eyes, with x-rays—the shape of the organism. Function of a living system is a description of that organism’s capabilities. The ability to respond to the environment is a function.
Today we know that life evolved on our planet between 3.5 billion to 4 billion years ago. The operative element on which life is based, carbon, perhaps, under specific conditions, could variate between forms. Darwinian selection was only possible once there was true heredity and inheritability. Selection amongst the earliest replicators led to the various species that inhabited our planet, today and in the past.
The evolution of an organism begins at the chromosome. There are slots along the chromosomes, call loci. Genes compete with alternate forms of genes, their alleles, for slots along the genetic locus. This gives rise to different phenotypic traits. The competition for genes for spots along the chromosome allow for mutation. These mutations are the essence of natural selection. As species don’t add to their genetic information during their lifetime, it is present within the gene pool and inherited upon birth. Changes in the conditions of the environment allow for animals of the same species to be ‘selected’ for survival. By selected I intend to mean that those with the genetic predisposition to survive the environmental change are born with the genetic basis for that survival and thus out propagate their contemporaries.
As we shall later see, our ancestors, far from ignorant, recognized the unity of all life–even of the non-living and the living–but did not, until much later, understand the manifold complexities of evolution under natural selection. The last generalization, having emerged in modern biology, I would like to give some mention, is all aspects of living phenomena, without exception, have a physiochemical basis. All properties of life can be understood by explicable laws of physics and chemistry. The earliest biologists didn’t recognize this, as Einstein didn’t prove the existence of the atom until 1906, a thought to which the ancient philosopher Democritus had given serious thought. Along with Boltzmann, in passing, the possibility of one constituent making up existing physical constructs of the universe.
We know, now, that living and non-living worlds are a part of a material continuum of physics, chemistry, and biology. The living world is still subject to the laws of physics and these same rules hold true in the non-living world. To the ancient philosophers, the relationship between the inanimate and the animate was ambivalent. One mode of thought, ‘Dust thou art to dust return,’ and it would seem, that such a remark, implies that living objects are related to non-living objects. Then there is the ‘pervasive element’ which was an early constituent of the metaphysical soul, and it was this that made living things different from inanimate matter. This is a concept incompatible with modern biology.
It would be unfair to blame our ancestors for their ambivalence. To understand living systems, we needed, first of all, the type of chemistry developed after the Renaissance. It did not, however, touch our science until this century. It would be pardonable to doubt if anyone could have done better during the ancient and early medieval period than in ancient India. It is disappointing for the biologist to see the uncritical reception of absurdities. Respecting the past ideals is admirable as they give perspective to our development as a species, but, to do them service, we must honor the spirit of their inquiry; we should not, by any means, discard contradictory, modern, testable knowledge for the ancient systems of religious tautology.
Despite occasional relapses into religious dogma, many of our ancestors were extremely accurate and perceptive people. It is this acuity of observation that provided the foundation for ancient Indian sciences. Nowhere is this capacity so explicit as, as observed, that relate to the structure of biological systems.
There is detailed knowledge of internal and external organs during the Vedic period. In Atharaveda, there is knowledge of the fallopian tubes, testicles, and semen. They knew more about the skeletal structure than just the bones; they knew about cartilage and ligaments. In the Caraka Samhita, we find that the number of bones in the human body amount to 360; we know, today, the number of bones in the human body are 206. It is more than likely that all of them were identified by Caraka’s time.
Susruta’s description, before the time of Caraka, of the anatomy of the human body, within the limitations of the human eye, is extraordinarily complex and analytical. The difference between vertebrates and invertebrates was, by Susruta’s time, widely known; ‘Some stand with the support of bone, others with muscles.’
Parasara, in the first century BC, added details to the understanding of the internal structure of a leaf. The description refers to ever smaller compartments, sap, and possibly the cell ‘wall.’ In ancient Hindu literature, the Brhadaranyakopanishad, for example, compares the human being with a tree; ‘A man, indeed, is likened unto a tree. His hairs are leaves, his skin the outer bark. His blood flows as does the sap from the wounded tree. Flesh corresponds to the inner-bark, his nerves are tough as inner fibers; bones behind the flesh are as the wood behind the tissue. The marrow of the bone resembles the pith.’
There is an element of realism in this kind of early poetry and makes for fascinating reading for students of ancient biology and the Vedic mythology from which it came. Susura systemized the classification of plants and animals into strict categories; categorical thinking is inherent in human thought, as one experience prepares one for another experience, and the recollection of each experience is put into a sort of recall-able category for when a situation demands it.
This is a human instinct, to categorize, and arrange into categories, as Susura did with the classification of plants and animals. 700 (plus) plants and 300 (or thereabouts) animals were referred to in the ancient Hindu literature (the biological history of which I am most familiar). They were classified in different ways: the basis of medicinal properties, utility, and morphology. Another attempt to classify animals in a systematic way can be found in one of the Upanishads; in the Chandogya, the designation was based on origin and development.
By these means of classification, there was as well a group comprising organisms born out of the moisture of the earth, such as gnats, lice, flies, bugs. It is of interest to think that, despite the size of these creatures, ancient biologists believed them to be menacing and dangerous, a scourge upon the Earth. It would be later on before the theory of ‘spontaneous generation’ was buried, once and for all, by Louis Pasteur, before we understood parasites and symbiosis, though not directly or immediately.
Taking this under consideration, our ancestor’s didn’t stray too far away from the mark in their attempts to understand and classify the living world. An elaborate classification of plants was made by Parasara; it was largely based on morphology and floral characteristics. Plants were classified into families and some of them mirror the families of today. Take for example: Leguminosea, the Crustacea, Cruciferae, Kapucynacea, Cucurbitacea, and Compositae. It is a tragedy that such classifications were not improved upon. The relationship, at that point in time, between various classes and orders was not analyzed. Had that been done, perhaps a more elegant and systematic classification could have emerged centuries ahead of Linnaeus.
It is a testament to the importance of dealing within many parameters at a time when classification is not a universally acknowledged subset of biological division, such as the groups, the phyla, the clade; and thus they never chanced upon the formation of variation, the cousinship between man and other animals, but they did well and, for their time, they were influential enough to bring more and more people into the biological sciences. We have these early teachers to thank for the likes of Charles Darwin and Gregor Mendel.
Combined, these two men, one a Victorian naturalist and the other a monk, transformed the way in which people looked at the animal world. It is through these glasses that I hope the reader will see. Knowledge of the universe is not beyond us; it may be ahead of us, but not impossible. Reverting to an adamant refutation of tested scientific truths does little for the instructor, other than annoy them, but does great damage to the person who, in adamant refusal, cannot see how grandiose and splendid nature truly is. I repeat this in a type of frustration. It’s like asking a blind child what his favorite color is.
We must not forever sweep that which we as of yet don’t understand under a comfortable carpet labeled, ‘God.’ This is not an answer; it is an admission of not knowing the answer. The answer, how it is formed and computed and synthesized, is the subject of the next chapter.