Mohsen Ezz El-Din Al-Bakri
3 hours ago
In the beginning, life on Earth was unicellular, reproducing asexually through division — that is, producing identical copies of itself. This method did not generate genetic diversity, which made organisms vulnerable to extinction under any environmental or pathological change. Later, some organisms began exchanging parts of their genetic material before dividing, which produced genetic diversity. At this stage, there was neither male nor female, but only slightly different cells capable of merging to form varied offspring.
Then a difference appeared between the gametes: some organisms began to produce large cells containing nutritional reserves — these later became eggs, and their ancestors became what we now call females. Other organisms produced small, fast-moving cells — these later became sperm, and their ancestors became what we now call males.
The large gametes are few in number, large in size, heavy, rich in nutrients, and precious for survival because they contain everything the offspring needs to survive after fusion. Their movement is very slow or almost nonexistent. The small gametes, on the other hand, are numerous, small in size, light, and fast-moving, but weak and unable to survive alone — they depend on the large gametes to complete the function of survival and reproduction. In short, the large one bets on quality and stability, while the small one bets on speed and quantity — and these two strategies are the origin of the division between male and female in nearly all sexually reproducing creatures.
At first, this was not a strict division; some organisms produced both types at once. Over time, specialization became highly beneficial, and the two evolutionary strategies became established: the “few, large, heavy, and valuable” strategy for the female, and the “many, small, light, and fast” strategy for the male. With time, these strategies evolved in most kingdoms that practice sexual reproduction — including algae, fungi, protists, fish, amphibians, and reptiles. In these multicellular organisms, a clear distinction between male and female began to appear: some individuals developed organs to produce large gametes and provide nourishment to offspring, while others developed organs to produce small gametes and transfer genetic material quickly. At this stage, a simple hormonal control appeared to regulate gamete production and reproductive functions.
Reptiles and birds branched off before mammals appeared; therefore, they retained only the basic skin glands and did not specialize in feeding the young. Among the ancestors of mammals, however, skin glands existed in both sexes in rows along the chest and abdomen for general purposes such as moisturizing, protecting eggs, or perhaps secreting substances or scents. Later, some of these glands evolved in females to secrete milk and feed the young. With this development, the rows were reduced to two central, functional glands for nursing in females, under the pressure of natural selection.
As for males, there was no selective pressure to preserve the old rows or to develop a nursing function. Thus occurred genetic streamlining — the elimination of unnecessary structures to reduce biological cost. Nevertheless, the original structural blueprint remained preserved in the genetic code, which is why they retain nipples with no function. In rare cases, more than two nipples can appear along what is known as the embryonic milk line — a condition called atavism, because it represents a temporary reappearance of an ancient ancestral trait.
These nipples exist in most male mammals, including cats, dogs, apes, and bats. In cats and dogs, the additional nipples or old glandular rows are often unnoticed because they are buried in thick fur — yet they exist anatomically. In some species such as horses and elephants, male nipples do not appear due to stronger embryonic suppression, but the structural origin of the glands still exists in the genetic code.
In multicellular embryos — now referring to mammals — the development of nipples begins in both male and female before sexual differentiation occurs. Then the sex hormones, such as testosterone in males, control the inhibition of breast tissue growth after the nipples have formed, which is why male nipples remain functionless.
In summary, the male and female were not two separate entities from the beginning, but gradually emerged from non-sexed unicellular forms. The difference between large and small gametes then evolved, followed by the emergence of specialized reproductive organs, then the evolution of mammary glands in mammals. Over time, evolution practiced genetic simplification, trimming away unnecessary ancestral structures in both males and females. What remains — the two nipples or their rare reappearance as atavism — is a trace of this long evolutionary history.
...
...
Regards,
always Giving
Mohsen
Then a difference appeared between the gametes: some organisms began to produce large cells containing nutritional reserves — these later became eggs, and their ancestors became what we now call females. Other organisms produced small, fast-moving cells — these later became sperm, and their ancestors became what we now call males.
The large gametes are few in number, large in size, heavy, rich in nutrients, and precious for survival because they contain everything the offspring needs to survive after fusion. Their movement is very slow or almost nonexistent. The small gametes, on the other hand, are numerous, small in size, light, and fast-moving, but weak and unable to survive alone — they depend on the large gametes to complete the function of survival and reproduction. In short, the large one bets on quality and stability, while the small one bets on speed and quantity — and these two strategies are the origin of the division between male and female in nearly all sexually reproducing creatures.
At first, this was not a strict division; some organisms produced both types at once. Over time, specialization became highly beneficial, and the two evolutionary strategies became established: the “few, large, heavy, and valuable” strategy for the female, and the “many, small, light, and fast” strategy for the male. With time, these strategies evolved in most kingdoms that practice sexual reproduction — including algae, fungi, protists, fish, amphibians, and reptiles. In these multicellular organisms, a clear distinction between male and female began to appear: some individuals developed organs to produce large gametes and provide nourishment to offspring, while others developed organs to produce small gametes and transfer genetic material quickly. At this stage, a simple hormonal control appeared to regulate gamete production and reproductive functions.
Reptiles and birds branched off before mammals appeared; therefore, they retained only the basic skin glands and did not specialize in feeding the young. Among the ancestors of mammals, however, skin glands existed in both sexes in rows along the chest and abdomen for general purposes such as moisturizing, protecting eggs, or perhaps secreting substances or scents. Later, some of these glands evolved in females to secrete milk and feed the young. With this development, the rows were reduced to two central, functional glands for nursing in females, under the pressure of natural selection.
As for males, there was no selective pressure to preserve the old rows or to develop a nursing function. Thus occurred genetic streamlining — the elimination of unnecessary structures to reduce biological cost. Nevertheless, the original structural blueprint remained preserved in the genetic code, which is why they retain nipples with no function. In rare cases, more than two nipples can appear along what is known as the embryonic milk line — a condition called atavism, because it represents a temporary reappearance of an ancient ancestral trait.
These nipples exist in most male mammals, including cats, dogs, apes, and bats. In cats and dogs, the additional nipples or old glandular rows are often unnoticed because they are buried in thick fur — yet they exist anatomically. In some species such as horses and elephants, male nipples do not appear due to stronger embryonic suppression, but the structural origin of the glands still exists in the genetic code.
In multicellular embryos — now referring to mammals — the development of nipples begins in both male and female before sexual differentiation occurs. Then the sex hormones, such as testosterone in males, control the inhibition of breast tissue growth after the nipples have formed, which is why male nipples remain functionless.
In summary, the male and female were not two separate entities from the beginning, but gradually emerged from non-sexed unicellular forms. The difference between large and small gametes then evolved, followed by the emergence of specialized reproductive organs, then the evolution of mammary glands in mammals. Over time, evolution practiced genetic simplification, trimming away unnecessary ancestral structures in both males and females. What remains — the two nipples or their rare reappearance as atavism — is a trace of this long evolutionary history.
...
...
Regards,
always Giving
Mohsen