Here’s how temperature-dependent sex could happen in humans.
The series «Imaginary Earths» speculates what the world might be like if one key aspect of life changed, whether related to the planet or humanity. The sex of humans is largely controlled by the X and Y sex chromosomes. However, in many reptiles and fish, sex is instead influenced by how warm or cool eggs are before hatching. What might life be like for humans if sex was likewise under the sway of temperature? The fact it was even possible to control the sex of animals using heat or cold was first uncovered in the rainbow agama lizard in 1966 by French zoologist Madeline Charnier at the University of Dakar in Senegal. She found hatchlings from eggs incubated at lower temperatures were female, while those that developed at higher temperatures were male. Since then, scientists have discovered other patterns of temperature-dependent sex determination. For instance, with the Hawaiian green sea turtle, females emerge if incubated above a certain temperature and males if below a certain temperature, and if temperatures in nests fluctuate between those extremes, a mix of males and females are seen, according to a 2020 study published in the journal Bionatura. In contrast, with the American alligator, females develop from extremes of hot and cold and males from intermediate temperatures. Related: What if humans had photosynthetic skin? Temperature controls sex determination, in all crocodilians, most turtles, many fish, and some lizards, according to organismal biologist Karla Moeller at Arizona State University. Within a specific window of time during the embryonic development of these animals, heat or cold can influence the production of sex hormones, which in turn can sway a hatchling’s fate. Moeller noted that one cause of temperature-dependent sex determination is an enzyme known as aromatase, which can convert male sex hormones to female sex hormones. In animals such as the red-eared slider turtle, heat during a specific developmental stage can increase levels of this enzyme, leading to more females. Evolutionary mysteries It remains uncertain exactly why these animals practice temperature-dependent sex determination, although a huge number of theories exist, Jennifer Graves, a geneticist at La Trobe University in Melbourne, Australia, told Live Science in a phone interview. «Our best guess is that temperature-dependent sex determination originated because reptiles do not have parental care and the eggs are in close interaction with the environment,» Diego Cortez, a biologist at the National Autonomous University of Mexico in Mexico City, told Live Science in an email.
