Epigraph:

“We created man from an essence of clay, then We placed him as a drop of fluid in a safe place, then We made that drop into a clinging form, and We made that form into a lump of flesh, and We made that lump into bones, and We clothed those bones with flesh, and later We made him into other forms ––glory be to God, the best of creators!––then you will die and then, on the Day of Resurrection, you will be raised up again.” (Al Quran 23:12-16)

“People, [remember,] if you doubt the Resurrection, that We created you from dust, then a drop of fluid, then a clinging form, then a lump of flesh, both shaped and unshaped: We mean to make Our power clear to you. Whatever We choose We cause to remain in the womb for an appointed time, then We bring you forth as infants and then you grow and reach maturity. Some die young and some are left to live on to such an age that they forget all they once knew. You sometimes see the earth lifeless, yet when We send down water it stirs and swells and produces every kind of joyous growth: this is because God is the Truth; He brings the dead back to life; He has power over everything.” (Al Quran 22:5-6)

Written and collected by Zia H Shah MD

Placenta is an amazing organ. Humans have created artificial heart and artificial kidney but are nowhere close to mimic the function of this miracle that we share with thousands of other mammal species.

The placenta in elephants is notably substantial, reflecting their considerable size and extended gestational period. In African elephants (Loxodonta africana), the placenta typically weighs between 9 and 21 kilograms, excluding the umbilical cord and membranes. ​ Elephant pregnancy is 22 months and a new born may weigh as much as 200 to 250 pounds.

Placental mammals, belonging to the infraclass Placentalia, are the most diverse group within the class Mammalia. They are characterized by the presence of a complex placenta, which facilitates nutrient and gas exchange between the mother and developing fetus during gestation. This group encompasses a vast array of species, including rodents, bats, primates, carnivores, and ungulates.

Number of Placental Mammal Species

Estimating the exact number of placental mammal species is challenging due to ongoing discoveries and taxonomic revisions. However, as of recent assessments:​

• The International Union for Conservation of Nature (IUCN) completed a comprehensive Global Mammal Assessment in 2008, documenting 5,488 mammal species. The majority of these are placental mammals, as they constitute the largest group within mammals.​
• A 2018 study published in the Journal of Mammalogy reported a total of 6,495 recognized mammal species, including 96 recently extinct species. This increase reflects new discoveries and refined classifications.​
• The Zoonomia Project, an extensive genomic study, analyzed the genomes of 131 placental mammal species, contributing to a better understanding of their diversity and evolutionary relationships.

​The placenta, a vital organ in mammalian reproduction, has an intriguing evolutionary history that intertwines with ancient viral elements. This narrative explores how retroviral integrations have contributed to placental development, leading to discussions on guided evolution.​

Retroviral Contributions to Placental Evolution

Approximately 25 to 40 million years ago, a retrovirus integrated its genetic material into the germ line of primate ancestors. This event led to the incorporation of viral envelope genes, notably those encoding syncytin proteins, into the host genome. Syncytins are crucial for the formation of the syncytiotrophoblast, a layer in the placenta responsible for nutrient and gas exchange between mother and fetus. Their origin from viral proteins highlights a remarkable example of molecular co-option, where viral elements are repurposed for host functions. ​

Mechanism of Syncytin Integration

Retroviruses possess envelope proteins that facilitate the fusion of viral and host cell membranes, enabling infection. When these viral genes became endogenous—integrated into the host germ line—they retained their fusogenic properties. In placental mammals, syncytins mediate the fusion of trophoblast cells to form the multinucleated syncytiotrophoblast layer, essential for effective placental function. ​

Implications for Guided Evolution

The integration of retroviral genes into mammalian genomes and their subsequent co-option for placental development raise intriguing questions about the mechanisms driving evolutionary innovation. Some argue that such events suggest a form of guided evolution, where external genetic material introduces novel functions that are naturally selected for their adaptive advantages. This perspective posits that horizontal gene transfer, facilitated by viral integrations, serves as a catalyst for complex trait development, potentially guiding evolutionary trajectories in ways that random mutations alone may not achieve.​

Conclusion

The evolutionary history of the placenta exemplifies how interactions between organisms and viruses can lead to significant biological innovations. The co-option of retroviral genes for essential reproductive functions underscores the complexity of evolutionary mechanisms and invites further exploration into the factors that guide the development of complex organs.

I do not have as much faith as the atheists to attribute all this amazing construction from a retrovirus to a mere chance or accident.