Consider the question: which came first, the chicken or the egg? This is the kind of puzzle that anyone can easily entertain. It has an aspect of humor. Yet when examined closely, we see it has the possibility of revealing something inherent about biology, about the nature of life. We’ll come back to this question.
There are two common forms of reproduction in nature. The simplest is replication, where a single-celled organism splits in half, creating an exact copy of itself. We have all seen a bubble that expands in size and then splits in two. This is the way that bacteria and other cells reproduce. The other form is sexual reproduction, where a male and female parent each contribute a set of genetic material that results in the creation of a unique off-spring.
Inside every cell of every living thing are chromosomes, which contain genes that are made of up DNA. Think of DNA as a mix of four basic ingredients that is used to make a recipe. Different species have different numbers and kinds of chromosomes. A fruit fly, for example, has only 11 chromosomes, which include DNA instructions for creating a pair of wings, sticky feet, and big eyes, while humans have 46 chromosomes, 23 from each parent. Human genes contain instructions for making different body parts, just as the fruit fly, but many more.
The human genome contains about 23 thousand genes. Strangely, the most number of genes belongs to the common water flea, Daphnia, a tiny creature about the size of a grain of rice. It has about 31 thousand genes. Biologists are not sure exactly why it has so many genes, but presumably it needs them to adapt to its difficult environment. Nature never fails to surprise.
Both males and females have special reproductive cells, or sex cells. The role of the male sex cell, or sperm, is to use its tail (or flagella) to swim to the female sex cell, known as the egg or ovum. Many thousands of sperm attempt to reach the female egg. If one of the sperm cells succeeds, it deposits its set of chromosomes inside the female egg. The female egg now contains both sets of chromosomes, her own, and the male set. With both sets of chromosomes combined, one from mom and one from dad, the egg can begin to grow inside the female.
There are many interesting details associated with the reproductive process, especially within the female body. For example, there is a chemical change in the immune system that allows the sperm cell, a foreign body that has entered the uterus, to continue along its way toward the egg without interference. Recent research has found that the ‘victorious’ sperm cell is held in a kind of ‘holding position’ before it is actually allowed to penetrate the egg. One of the unanswered questions in biology is: how do the egg and sperm recognize one another amongst all the other cells?
In humans, the development from the tiny fertilized egg to an infant (the way a baby looks when it is born) takes about 9 months. In other creatures, the length of a female pregnancy differs widely. Usually, a larger creature means a longer pregnancy. A baby elephant grows inside of its mother for 22 months before it is born, while a mother flea is pregnant for only 11 days.
Human males produce millions of sperm cells over a lifetime, while a female deposits about 400 eggs in total, one egg every month from the time she reaches puberty until she is middle-aged. A high school biology teacher became famous for her favorite phrase “sperm are cheap.”
The chance of any of us being born is astronomically slim. We are the chosen ones. Life is a gift, a miracle. As filmmaker Woody Allen has famously put it: ‘80 percent of life is showing up.’ Many are called. Few are chosen.
In medicine and biology, the word ‘SEX’ is commonly used to refer to a male or female. In social science and cultural anthropology, male and female are words synonymous with gender. In fact, the definition of sex is the creation of ‘a new organism containing genetic material from more than one parent.’ Creatures tend to mate strictly with members of their own species. Gender refers to ‘differences between any two complementary organisms capable of mating.’ ‘Many species contain hundreds, even thousands of genders.’ (see Lynn Margulis and Dorian Sagan’s brilliant and informative book What is Sex?)
There is a great range of sexual behavior in the plant and animal kingdoms that includes heterosexuality, homosexuality, bisexuality, and non-reproductive sex. Through the process of natural selection, sex and gender are ‘adjusted’ to whatever behavior is required for a species to survive. Sexual diversity and gender variation are widespread. As humans, we are accustomed to thinking that there are two genders, male and female. Some species of plants include thousands of gender differences. Many species of fish change genders routinely. Shrimp, for example, are born male and later change to female.
Homosexuality, transvestism, sex change, and so-called ‘sneaking’ are alternative reproductive strategies that reappear throughout the animal world. Single-sex pairing is common in both males and females. Sometimes there aren’t enough males or females to go around. Or in the case of insects, young males may learn courtship behavior from older males. There are many examples. Transvestism involves female mimicry, especially males who sneak into the territory of males posing as females. (see biologist Joan Roughgarden’s fascinating book Evolution’s Rainbow)
There are also different forms of asexual reproduction. Parthenogenesis, or Virgin Birth, for example, is found in females, where growth and development of embryos occur without fertilization by a male.
In another example, there are many hermaphroditic species containing individuals that can reproduce by themselves because they contain reproductive organs of both sexes in a single individual’s body.
There are numerous examples of asexual reproduction throughout the natural world.
There are many unanswered questions regarding sex and reproduction. For example, when and where, exactly, did the first independent male and female genders originate? A place to begin might be with the process of symbiosis in single-celled creatures. Perhaps with the combination of genes acquired from two different organisms, there would have been enough genetic variation in the resulting generations of offspring to develop ‘specialized’ genders such as male and female. Given the relationship between sex and death, this would be nearly assured.
Here is another interesting question: when did humans first discover that sexual behavior is the mechanism for procreation? It wasn’t until about 1900 that we knew of the existence of sperm cells. 10, 000 years ago we had already domesticated plants and animals and were practicing animal husbandry, so we certainly understood cause and effect by then. 40,000 years ago, with the advent of modern humans, we were recording the cycles of the moon, which are clearly correlated with menstrual cycles. Were our early ancestors naive, or did they understand the biological reality of sex and reproduction?
Today in hospitals and clinics all over the world, there are new methods and techniques dedicated to biological reproduction never imagined even a century ago. Modern technology has made it possible to remove an egg from the mother, fertilize the egg with father’s sperm cells in a petri dish in the lab, then reinsert the fertilized egg into the mother’s body, followed, hopefully, by a normal pregnancy. This form of assisted reproduction is known as in vitro fertilization, and has become a popular means of achieving parenthood for women who don’t have access to natural pregnancies for one reason or another.
Startling new medical research has shown that along with the typical 400 eggs produced in a lifetime, women also have a previously unknown store of ovarian stem cells from which new eggs can be created. Experiments have shown that ovarian stem cells can be isolated and made to grow into mature, healthy eggs that are capable of fertilization and reproduction. The consequences of this research are enormous both for women looking to reproduce through in vitro fertilization and for extending their reproductive lives.
Laboratory cloning is another form of assisted reproduction. Cloning is simply the process of making copies. Twins are clones. Because we have modern techniques for replicating and modifying DNA, science is able to ‘make’ or re-create plants and animals in almost any form. A variety of foods, along with small and large animals, have been cloned using various laboratory methods for nearly two decades. In years to come, human-assisted reproduction will change the way we experience life and family.
Today, with assisted reproduction, the creation of body organs and genetically modified life forms in the laboratory, genetically modified foods already on the shelf, and the possibility for human cloning within our grasp, we face difficult moral and ethical issues that challenge the very nature of who we are, and how we manage our ability to create and modify new and existing life.
Back to the question of the chicken and the egg. It has been pointed out that the Egg has existed on Earth for at least several hundred million years, while the Hen has been here only fifty million years. It seems obvious that the egg must have come before the chicken.
However, in direct opposition to this idea, it is possible that proteins may have evolved prior to DNA molecules. Proteins make up the structural framework of a cell, while DNA is typically located inside the cell’s nucleus. If you think of the nucleus as the embryo or egg, and proteins as its protecting membrane, or material body, then one may conclude that the chicken, did in fact, come before the egg.
view related Prose Poem that can be read aloud:
for many colorful and idiosyncratic examples of sex and reproduction, see:
Marlene Zuk’s book, Sex on Six Legs, Houghton Mifflin, Harcourt, 2001