For that which befalleth the sons of men befalleth beasts; even one thing befalleth them: as the one dieth, so dieth the other; yea, they have all one breath; so that a man hath no preeminence above a beast: for all is vanity. All go unto one place; all are of the dust, and all turn to dust again.

Ecclesiastes 3:19–20

No honest human is free of self-doubt about the validity his or her convictions, and the manner in which one deals with self-doubt expresses a degree of personal integrity. Fortunately for the theory of evolution, Charles Darwin (1859) was no exception from this rule, and it is not surprising that he started to doubt the universal applicability of natural selection—a concept he defined—a mere year after publishing The Origin of Species. In a letter to a friend, he stated, “[T]he sight of a peacock’s train whenever I gaze at it makes me sick” (Darwin [1882] 1993). What was making him sick was the doubt and not the sight itself, because he found peacocks’ tails to be pretty. In The Origin of Species, Darwin developed the concept of speciation of living organisms, describing it as a gradually emerging outcome of natural selection over many generations as separated groups of individuals of an ancestor species get exposed to a sustained environmental pressure. In such a situation, individual animals that manage to produce descendants before they die show themselves to be a good fit to their environment. Moreover, with the environment being relatively static and heredity working as it does, this also meant these descendants would probably meet the demands of their environment—that is, be suited to the environment that “selected” their ancestors. The concept of selection was well known to Darwin from looking at manipulations farmers did with domestic animals. It was, and still is, a custom for the breeders of horses, dogs, cows, etc. to carefully select specific pairs of animals for mating because they have desirable traits their progeny will likely show as well. The workings of human-enforced selection and natural selection are both efficient in changing the distribution of hereditary traits in a species. The major difference between the two is that humans select traits they want in animals within a relatively short time, while nature applies its pressure over many millions of years. Extrapolating from this, Darwin concluded that the average appearance and behavior of any specific species at any specific moment are an outcome of a lengthy selection process; this also meant that each species living in nineteenth century existed because in was (still) in harmony with its surroundings. And yet, there were peacocks. A peacock’s train is blatantly inconsistent with the concept of natural selection, for what natural advantage can there be for a male peacock to be burdened with a heavy and clumsy tail in a world full of predators? This irksome fact stimulated Darwin to think about all possible forces of selection and helped him discover the principle of sexual selection. Consequently, in 1871, he published his second book, The Descent of Man, and Selection in Relation to Sex. In it, he postulated that mating success is itself a source of selection pressure and explained the male peacocks’ tails as a key feature that attracts female peacocks. Thus, for a male peacock, the risk of falling prey to a fox before procreation is offset by a much increased likelihood of fathering peacock chicks before the fox snatches him. Darwin still could not guess what may be going through a female peacock’s mind when she sees several eligible males, and he bungled things a bit by talking about beauty as humans see it as one of the bases of sexual selection. Even among the naturalists of the time, it was not uncommon to anthropomorphize animals, so Darwin had relatively few conflicts with other biologist about this issue. Subsequent generations of biologists thought differently, and this reliance on aesthetics made many suspicious about sexual selection as a concept. Few humans find beauty in either sex of cockroaches, yet they clearly mate with each other and leave abundant offspring. Likewise, we fail to appreciate the beauty of placozoa, the simplest multicellular organisms that occasionally reproduce sexually (Signorovitch, Dellaporta, and Buss 2005). Nevertheless, regardless of the “beauty principle,” the notion of sexual selection survived, and the appeal of male peacocks’ tails was revisited in the twentieth century. In a series of experiments, different aspects of the male peacock’s tail were obscured and others left untouched to find out what exactly is attractive to female peacocks. It turned out to be the number of “eyespots” on tail feathers and not the size of the tail (Petrie 2021). Why exactly the female peacocks value the “eyespots” is still not known, but Marion Petrie explains that the existence of variance in mating success opens possibilities for rapid evolutionary changes that benefit species as a whole.

Even though biologists from the 1800s did not do peacock experiments, they observed the nature around them avidly and made numerous exciting and wide-sweeping discoveries. Biology of that time was exceedingly popular because of easy access to a new gadget, the microscope, and things to look at under it such as pond water. This situation was so rife with opportunities for discovery that many talented amateurs made their names in this field of science. Darwin was the great pioneer of biology at that time, but some of the other enthusiasts found fault with him and his theories. For example, a book review of The Descent of Man, and Selection in Relation to Sex published by biologist St George Jackson Mivart (1871) concluded with the following:

Without a sound philosophical basis, however, no satisfactory scientific superstructure can ever be reared; and if Mr. Darwin’s failure should lead to an increase of philosophic culture on the part of physicists, we may therein find some consolation for the injurious effects which his work is likely to produce on too many of our half-educated classes. We sincerely trust Mr. Darwin may yet live to furnish us with another work, which, while enriching physical science, shall not, with needless opposition, set at naught the first principles of both philosophy and religion.

In a nutshell, even though all the most important biological discoveries of the nineteenth century showed, in some form or another, that all life on Earth functions under the same principles, Darwin’s explicit statement that humans are part of the animal kingdom found disfavor with many of his religious contemporaries. Interestingly, even though he hoped to protect “the first principles of religion” and thus distance himself from Darwin, Mivart eventually got excommunicated from the Catholic Church for some unusual ideas about hell and ended up buried in a dissenter’s cemetery for four years after his death. Meanwhile, Darwin was buried in Westminster Abbey, one of the most important Anglican churches, with pomp and circumstance.

Despite differences of opinion, both Darwin and Mivart viewed themselves as philosophers as much as biologists. Engaging oneself in a philosophical way of thinking was something most scientists in the 1800s did as a matter of course, and the philosophy of biology was not seen as remote from other philosophies, including the philosophy of religion. It is therefore perhaps surprising that Darwin’s contemporaries focused on the philosophy of religion did not attempt to reach after new vistas of thought opened by biological philosophy. Fortunately, that is no longer the case. Today, many of those who study the philosophy of religion are eager to consider philosophies of “natural sciences” when developing new positions. One of the intentions of this article is to provide something like a table of contents of biology and encourage non-biologists to pick biological concepts worthy of contemplation that would appeal to them.

Darwin’s understanding of biological evolution opened human minds to thinking about the outcomes of survival under the pressures of the natural world, but our understanding of life owes as much to two other nineteenth century biologists: Matthias Jakob Schleiden and Theodore Schwann—originators of cell theory. In fact, there are some who argue the cell theory enabled Darwin to think about heredity in a way that helped him conceptualize evolution (Müller-Wille 2010). These two German scientists were passionate microscopists and friends who realized cellularity is universal for plants and animals alike (Hajdu 2002; Charpa 2003). It is true that the microscope was discovered before their time and that others coined the word “cells” and discovered cell nuclei, but nobody before them dared to say that every living creature is made of cells. In 1838, Schleiden published his understanding focused on plant cells, while Schwann formulated a universal cell theory in 1839. Their book, translated into English in 1847, states succinctly and accurately: “All living things are composed of cells and cell products” (Schwann and Schleyden 1847). Interestingly, as a devout Catholic, Schwann asked the Archbishop of Malines for a blessing for his book on cellularity, and received it. Whether the archbishop, or even Schwann himself, understood the full implications of cell theory is uncertain. The fact is that while the theory of evolution links humans with primates, the universal cell theory connects humans with every other living creature on the planet. Despite the differences in the size and shape of their cells, all living organisms manage their existence using the same approaches to synthesize proteins and nucleic acids; all engage in some type of cell-to-cell fusion. We are everyone’s distant cousins and related to peacocks and plants as well as placozoa and bacteria.

Because a cell is an incredibly complex structure and includes the same key components and parts that function in the same way in all living organisms, an undisputed consensus in biology is that all living organisms have a single common ancestor. This would be a primordial species of single cell organisms that lived billions of years ago, having its entire hereditary material—deoxyribonucleic acid (DNA)—organized into a single piece, a single chromosome. Importantly, single cells that were the individual members of this species were not too fastidious about the separation between self and other cells and often merged with each another and exchanged cellular materials. As a matter of fact, some of the single cell organisms living on Earth today still engage in unions with other single cells even if they are not of the same species. For example, individual cells of two different species of bacteria from Clostridium genus have been found to engage in direct cell-to-cell connections. In these interactions, they exchange their intracellular material, which then leads to changes in metabolism. Sometimes, as a result of material exchange, the hybrid cells start to produce non-native metabolites. and it is this unexpected change of metabolic behavior that allowed the discovery of the phenomenon of exchange (Charubin and Papoutsakis 2019). While in this instance material exchange did not lead to hereditary changes, such changes occur easily when there is an exchange of genetic material between cells of the same species. Bacteria Escherichia coli, one of the regular inhabitants of human intestines, engages in two different types of “mating.” In these processes, bacterial cells mix their hereditary material and become genetically unstable due to the increased amount of DNA. When the DNA amount gets in balance again, the single bacterial chromosome is often refashioned as a combination of the two initial DNAs (Gratia and Thiry 2003). While temporary genetic instability still affects bacteria that are exchanging genetic material, the descendants of primordial cells developed a new way to handle their DNA that stabilized it and laid the ground for still more frequent mixing of hereditary material. This was the development of the nucleus—a cellular sub-compartment where the hereditary information of a cell, its DNA, is stored. This early nucleated organism gave rise to all organisms with true nucleated cells, the so called eukaryotes, via an ancestral nucleated cell that is referred to as the last eukaryotic common ancestor (LECA) (Goodenough and Heitman 2014). LECA preceded the separation between species that committed to the consumption and metabolism of ingested materials and became fungi or animals and species that adopted the ability to absorb the energy of the sun and became plants. LECA is also believed to be the organism that first developed sexual reproduction. Subsequently, this type of reproduction developed different specific manifestations in different descending species.

At first glance, considered purely in terms of efficiency, sexual reproduction is a needless complication if the thriving of a species is the only question under consideration. The majority of single cell organisms reproduce asexually, simply by splitting into two daughter cells. While that is not numerous offspring for a single individual, and more offspring can be generated by a multicellular organism, duplications of single cell organisms are, nevertheless, guaranteed duplications. If we look at the entire populations of individuals of single-cell organism species, their populations can double in size in a single generation. This is an efficient process for growth and is responsible for the fact that the calculated current biomass of the simplest and smallest single cell organisms—bacteria—accounts for 10 % of the entire biomass on the planet, while humans account for 0.06%, and all the combined wild animals and birds for less than 0.01 % (Bar-On, Phillips, Milo 2018). This makes sense considering that, unlike bacteria, many individual multicellular organisms do not generate any offspring even when, in theory, they could be a parent to thousands. The difference in reproduction strategy between single cell and multi cell organisms is also interesting to consider from the perspective of death of an individual. While a multicellular organism may or may not reproduce, it is certain that it will eventually die. A single cell organism, on the other hand, literally dies for and lives through its progeny. Each single cell bacteria and its largely identical sister cell are born through cell division of the mother cell; when such a division is done, nothing of the mother cell remains.

Before we go further into the explanation of the sexual life cycle, it is useful to explain that regular cell division—the way in which a cell duplicates itself—is a process through which genetic material first doubles and then gets equally divided, transmitting the identical information to the two daughter cells. This type of cell division, by which a parent cell ceases to exist and two of its daughter cells come to be, is called mitosis. In any multicellular organism, most of the cells that make up its body mass are generated by this process—a cell divides into two daughter cells, which each divide into two more cells, and so on—and a body grows. This form of cell division does not, however, support sexual reproduction. Existence of the sexual life cycle depends on coincidence of two different processes that most likely developed simultaneously, complementing one another for the disadvantages to the organism either process on its own could cause. One of the two processes—cell fusion and material exchange between cells—was briefly mentioned earlier in the context of cell–cell interactions. It should be noted that in the context of bona fide sexual reproduction, accuracy of cell fusion is of paramount importance, and intricate mechanisms have developed for mutual recognition of reproductive cells with the complementary DNA sets (Goodenough and Heitman 2014). The second process necessary for sexual reproduction is a new type of cell division—division that happens twice in immediate succession but with only one round of duplication of genetic material. This process is called meiosis, and at the end of it, a parent cell has disintegrated into four daughter cells, each with only one half of the genetic material and incapable of further division. Cells called gametes that form the basis of sexual reproduction fall into this category. Once generated, they will either participate in merging with a compatible gamete cell or die as they were. The cycle of sexual reproduction depends on the generation of gametes and their fusion. At the end of gamete fusion, a single cell with a full complement of genetic material is created. This newly minted cell—a zygote—can now go through the process of mitosis repeatedly and grow into a new organism. If the zygote belongs to a species of Placozoa, the process is over after two divisions, because Placozoa are organisms made of only four cells. If the zygote is human, it will first split into eight unconnected cells generated through three mitotic divisions. These eight cells will then start to form permanent cell-to-cell connections. These connections are the basis of multicellular organisms functioning as a single organism (separation of cells before this stage can lead to twinning), and they are necessary for subsequent coordinated mitoses that allow for the development of a baby over the period of nine months. It is beyond the scope of this article to talk about embryology, but it is worth mentioning that the correct development of the majority of multicellular organisms includes not only programmed cell divisions but also programmed cell death. The fact that cell death is hardwired into the successful growth of multicellular organisms echoes the fact that both life and death of individuals are necessary for the continuation of life for their species.

The necessity of death for a successful propagation of life is one of the biological concepts that can provide ample room for philosophical reflection and deep religious contemplation. The emotional and psychological turmoil that follows bereavement tears at families and communities, and every world religion has developed approaches for helping the bereaved cope with their loss. A firmly established understanding of death as necessary for the existence of all life that exists and has existed could provide solace in a moment of crisis. For most of us, the greatest moments of anguish derive from an expectation that we should be exempt from what befalls others. Seeing ourselves as members of a species and temporary representatives of all life on Earth is not emotionally or psychologically impossible. Biology provides us with daily confirmations of this fact, and religions focused on the emotional stability of their flock could capitalize on it much more than they do.

Sexual reproduction is, amusingly enough, often referred to as a problem, by which it is meant that it is difficult to justify its existence. The essence of this “problem” is sometimes expressed as follows: If a species is comprised of asexual females (in this case meaning simply an individual capable of delivering offspring), every living individual of that species will be equally capable to produce offspring. But, if a specific species is sexual, then only the female half of the individuals will be capable of delivering offspring. Because of that, the length of survival of the sexual females must be double that of her asexual peers to produce the same number of offspring (Petrie 2021). In short, the reproductive success of sexually reproducing species versus that of an asexual species identical in every other respect requires two times greater fitness of females. Regardless of whether this is a real-life paradox or a mathematical one, like those developed by Zeno, researchers of the twentieth and twenty-first centuries who tried to uncover the universal benefits of sexual reproduction did not meet definitive success (de Visser and Elena 2007; Otto 2008; Goodenough and Heitman 2014). It appears the fitness benefits of sexual reproduction may be many, but they are all relatively subtle. Regardless, the facts remain that almost all multicellular organisms reproduce sexually and that mutations that lead to asexuality get adopted by very few species (Otto 2008). Even though we are still unable to express it by a single and simple mathematical model, sexual reproduction increases the fitness of the living species and their likelihood of survival.

Clearly, once sexual reproduction came into being, the possibility of sexual selection also arose. Summarizing the available knowledge about sexual selection, a researcher who has worked with peacock tails said, “[S]exual selection is profoundly different from natural selection because, uniquely, it can simultaneously promote and maintain the genetic variation which fuels evolutionary change” (Petrie 2021).

In other words, sexual reproduction coupled with sexual selection may ensure a species has a latent capacity to face the pressure of natural selection, even when such pressure changes intensity or direction. In essence, sexual reproduction and selection come with genetic baggage that ensures a wide genetic variability among the newborns. If the environment has not changed since the parents were born, progeny similar to the parents will thrive the best. If the environment has changed, however, some of the genetic outliers from the next generation may be better suited to the changed environment. Importantly, these individuals will still also retain the genetic material that was better suited to the previous natural circumstances. Therefore, should the environmental changes go through reversals, sexually reproducing organisms will have a good chance to cope with them. Unidirectional changes of the environment, on the other hand, are more likely to overwhelm the genetic adaptability of a species, regardless of its type of reproduction, as can be seen from fossil records.

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