Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
The word sex comes from the Latin secare which means to separate or divide. Most one-celled organisms form offspring through asexual reproduction. It is asexual because there is no separation of genetic material. Therefore divided chromosomes do not need to be brought together and the new life produced is genetically identical to the original.
In contrast, most multi-cellular organisms form offspring in a much more complicated way called sexual reproduction. In humans, each of the chromosomes making up the 23 pairs containing the genetic material for life is separated from its partner and placed into gametes called male sperm and female eggs. Human reproduction involves sexual intercourse in which the male deposits sperm in the vagina of the female so they can move into the uterus and one of them can join its separated 23 chromosomes to the ones within the female egg in a process called fertilization. The fertilization of the female egg by the male sperm results in a one-celled zygote which then contains the full complement of genetic material and is distinct from its parents.
As difficult as it may be for evolutionary biologists to explain the blind and unguided development of the different organ systems and the body’s ability to control them, because of the mechanism humans must use for reproduction they must also explain the simultaneous development of both males and females since neither is of any use to propagate the species without the other.
The three things needed for a human to reproduce are: (1) either be male or female and have all of the right sexual parts, (2) be able to produce enough sperm or release an egg into a fallopian tube, and (3) be able to participate in sexual intercourse so sperm is released into the vagina or have a clear path for sperm to swim toward the fallopian tubes.
In my last article, I showed that for the first few weeks of life the human embryo is asexual. That is because the primordial gonads have not declared themselves to be testes or ovaries yet. Notwithstanding where the embryonic structures that develop into mature sexual organs came from, the human embryo is destined to become female by default unless acted upon by specific chemicals. These include the Testis Determining Factor (TDF) (usually found on the Y chromosome), the enzymes needed to convert cholesterol into testosterone and testosterone into dihydrotestosterone, the androgen receptor on the Wolffian ducts and the tissue that will become male external genitalia, Anti-Mullerian Hormone (AMH) and the AMH receptor on the Mullerian ducts.
If all of these chemicals are present and working properly the human embryo will usually develop into a normal male. But if the TDF is absent it will usually develop into a normal female. However, if the TDF is present, directing the primordial gonads to become testosterone-producing testes, but the androgen receptor is absent or defective (Complete Androgen Insensitivity Syndrome (CAIS)), the result in an XY female. XY females occur in about one in twenty thousand “male” births and have testes instead of ovaries, no genital duct system and female external genitalia. So for the first decade of life they look like normal little girls. However, experience teaches that although humans are sexually differentiated as male or female at birth, they are not able to reproduce. Most children begin to show signs of their sexual development to come by the end of the first decade. Over the following years they will undergo sexual and bodily development in a process called puberty. Puberty is a constellation of physiological changes that, except in, for example, XY females, not only enables human beings to reproduce but also prepares them for their natural role in the family. Let’s consider how this happens.
The hypothalamus and pituitary work to control many different vital hormones in the body. For example, the hypothalamus secretes Growth Hormone-Releasing Hormone, which stimulates the pituitary to release Growth Hormone (GH). As its name implies, GH is very important in the overall growth and development of the body. The hypothalamus also sends out Thyrotropin-Releasing Hormone (TRH), which tells the pituitary to send out Thyroid Stimulating Hormone (TSH). It is TSH that controls the production of thyroid hormone from the thyroid gland, which mainly affects the body’s metabolic rate. Furthermore, the hypothalamus sends out Corticotropin-Releasing Hormone, which tells the pituitary to send out Adrenocorticotropin Hormone (ACTH). ACTH stimulates the adrenal glands to produce cortisol, another hormone that is important in the body’s metabolism, in addition to androgenic hormones.
The hypothalamus and pituitary together control the production of a hormone such as thyroid hormone, or others, through a process called feedback inhibition. For example, the hypothalamus and the pituitary have specific receptors that allow them to sense the blood level of thyroid hormone. If it rises above what is needed the hypothalamus reduces its output of TRH and the pituitary lowers its output of TSH. The feedback of the thyroid hormone level in the blood serves to inhibit the release of TRH and TSH to maintain control of the blood level of thyroid hormone.
The production of the sex hormones is regulated in the same way by the hypothalamus and the pituitary. The hypothalamus secretes Gonadotropin-Releasing Hormone (GnRH) which attaches to specific receptors on certain cells in the pituitary and tells them to send out the gonadotropins, Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). It is FSH and LH that attach to specific receptors on the testes or the ovaries to produce the sex hormones, testosterone and estrogen, respectively.
In the first decade of life it appears that the hypothalamus and the pituitary are very sensitive to the feedback inhibition of the sex hormones. This means that prior to puberty, very low levels of testosterone and estrogen are able to prevent the hypothalamus from releasing its GnRH and the pituitary its FSH and LH. This results in the blood levels of testosterone and estrogen being very low prior to puberty.
A few years before puberty the adrenals increase their output of androgenic hormones which causes a small growth spurt and the development of pubic and axillary hair. What actually triggers the beginning of puberty is, as yet, poorly understood. However, what is known to happen is that the hypothalamus and pituitary start to become progressively less sensitive to the sex hormones. The gradually diminishing feedback inhibition of the sex hormones on the hypothalamus and pituitary results in their slowly increasing their output of GnRH and the gonadotropins, FSH and LH, respectively. By the time puberty is in full swing, the levels of gonadotropins and the sex hormones have risen significantly.
During male puberty, more FSH and LH attach to specific receptors in the testes and cause an increase in testosterone production, while at the same time giving them the ability to produce sperm. Puberty in the male also results in the progressive increase and coarsening of facial, chest, axillary, abdominal, extremity, and pubic hair, with enlargement of the vocal cords and deepening of the voice. Moreover, with the associated increase in the pituitary output of Growth Hormone (GH), the male experiences a significant linear growth spurt and the development of his musculoskeletal system as well. Furthermore, along with the capacity for sperm production, puberty brings on enlargement of the penis, scrotum, and testes. Finally, testosterone not only plays a major role in sexual differentiation, development, and maturation, but also in the desire for sexual relations. In addition, testosterone is important in giving the male the ability to maintain an erection for adequate penetration into the vagina and ejaculation during sexual intercourse. All these developments prepare the boy to become a man and later a father.
During puberty in the female, more FSH and LH attach to specific receptors in the ovaries and cause an increase in estrogen production while at the same time giving them the ability to develop an egg. Puberty in the female results in an increase in mainly pubic and axillary hair that is not as coarse as in the male. Moreover, breast development takes place so that the potential mother will be able to provide breast milk for her infant. In addition, an increase in Growth Hormone (GH) results in a significant linear growth spurt and development of her musculoskeletal system as well. Puberty also brings on enlargement of the external genitalia and increased mucous production within the vagina and uterus. Finally, along with the capacity for egg development, the increase in FSH, LH, and estrogen allows for ovulation, where the egg is released and can enter the fallopian tube.
Inside the fallopian tube the egg can meet and join with the sperm, which has been deposited into the vagina by the male during sexual intercourse, to form new human life. After ovulation, the ovaries mainly secrete the pregnancy hormone called progesterone. The estrogen before ovulation, and the progesterone after ovulation, attach to specific receptors in the lining of the uterus to make it thicken up and produce more mucous helping it to prepare for pregnancy. If a pregnancy does not take place, the gonadotropins (FSH, LH) and the female sex hormone levels (estrogen, progesterone) drop precipitously to cause the lining of the uterus to shed in the process called menstruation. The first menstrual period generally marks the beginning of female fertility and usually takes place on a monthly basis for the next thirty or forty years.
All these developments prepare the girl to become a woman and later a mother. However, during puberty an XY female will develop normal breasts and in every way look like a normally maturing woman, except when she fails to menstruate. Investigation will then uncover her as yet unknown (even to her) secret.
Remember that an XY female has Complete Androgen Insensitivity Syndrome (CAIS). In this case, the testes form testosterone, but since the androgen receptors are absent or not working, the Wolffian ducts degenerate and the external genitalia become female. However, in addition to testosterone the testes also produce Anti-Mullerian Hormone (AMH), which attaches to specific AMH receptors on the Mullerian ducts and makes them degenerate as well. So the XY female has neither a male nor female genital duct system and her vagina ends in a blind pouch. If an XY female has testes instead of ovaries how can she develop breasts during puberty?
The answer lies in the fact that breast development does not, per se, depend only on estrogen but rather the ratio between estrogen and testosterone. Since during puberty the normal XY male produces lots of testosterone and relatively small amounts of estrogen, his breast tissue does not develop. During puberty the normal XX female produces lots of estrogen and only relatively small amounts of testosterone, so her breast tissue does develop. Although an XY female has testes that produce lots of testosterone, because there are no androgen receptors for it to take effect, this allows the small amount of estrogen she produces to dominate and cause breast development. In fact, without any androgenic effects in their bodies, XY females are some of the most femininely attractive women in the world.
As noted above, the human embryo is destined to become female by default unless several other chemicals swing into action to make it become a normal male. But that’s only part of the story because for the first several years of life, humans, whether male or female, cannot reproduce. Puberty first involves an as yet unexplained reduction in feedback inhibition so there can be a significant increase in the release of GnRH from the hypothalamus and FSH and LH from the pituitary. This also requires the presence of specific receptors on their target tissues. It leads to the testes being able to produce sperm and more testosterone and the ovaries being able to release an egg and more estrogen (and progesterone) so males and females can reproduce.
It is information, wrapped in chemical signals, that makes the embryo become male or female and initiates puberty, thus making human reproduction possible. As Stephen Meyer observes in the Discovery Institute film The Information Enigma, the major discovery of the last half of the 20th century was that it is information that drives biology. In generating information, all human experience points to a mind rather than a random and unguided material process.
But clinical experience teaches that just having all of the parts present for reproduction doesn’t automatically guarantee fertility and the natural ability to bring about new human life. That’s what we’ll begin to explore next time.
Photo credit: © aallm — stock.adobe.com.