Before I go anywhere, I just have to note again that the embryos above are only approximately at developmental level (or Carnegie stages). I worked specifically from photos, and it was hard to pin down each one at a specific stage, which resulted in a game I’m not 100% happy with. The version of the game on the Exploratorium website makes it much more challenging. Secondly, I’m aware of Ernst Haeckel. Ah– Stop. Stop it. Stop writing that comment right now. I’ll address herr Haeckel in a later comic, when I have time to give his story the proper treatment. Now then, shall we?
Nadir’s comic last Friday—in the context of the human navel—reminded us that not every trait has an adaptive explanation. First described by Stephen Jay Gould and Richard C. Lewontin, it is commonly used to tackle the famous question: Why do men have nipples? The answer lies far back, sometime after we were a blob of cells (which would have made the above game much more difficult) but before we look like a human being.
When a sperm cell meets an egg cell, and they really love each other, they form a zygote. This cell begins to divide in a process known as cleavage, called this because at this stage, it does not grow in size. The zygote splits into enough cells to form a blastocyst, and things get interesting. To understand embryo development, and why most animals look the same early on, one has to understand that human DNA is very similar to the DNA of every other animal on the planet (this will also be covered in another comic). So how do we get so different?
Embryonic stem cells inside the blastocyst contain all the information needed to create the entire organism. Different cells don’t have different DNA, just different gene expressions. Think of the DNA chain like a blueprint, and as the blueprint is read, different switches are turned on or off. This is called cell signaling. Due to our genetic similarity to our animal relatives, this starts us off all looking remarkably similar, with tails and little nubs for limbs. But later instructions can override these initial similarities: Placental mammals begin life with a yolk sac, which disappears early on; our tail diminishes—leaving only the coccyx; whales and dolphins (letter D above) start life with hind leg nubs, which are re-absorbed into the embryo later, and like the tailbone, some vestigial traces remain. As each stem cell changes into another type of cell, so those cells can branch off again into more types, creating more specific features as the animal grows.
Similarly, the differences between sex happen quite late in the process. The first signs begin to appear about 11 weeks in, and are distinguishable via ultrasound and the super-serious method that doctors like to call, “the angle of the dangle.”* This is serious.
All animals begin life as female**. While the sex of the embryo is determined based on its chromosomal makeup, it appears as an androgynous worm-blob until the genes that code for primary sexual characteristics kick in. Only then do the sexes begin to differentiate. But like the coccyx or the whale hind-leg bones, clues of our unisex origins are left upon our adult features—plain to see without getting into the graphic nitty-gritty (kids, ask your parents about the scrotum seam).
So what’s with the can in the title? Well as I was looking up images of embryonic development, I learned the sordid tale of Ernst Haeckel’s infamous embryology print. I know that even suggesting that all embryos look alike, I invite a wave of creationists here, like seagulls attracted to a Haeckel strawman. I’m preparing for the worst.
P.s. In case you missed the link earlier, you really should check out The Multidimensional Human Embryo site.
This post was brought to you by Neon (Ne).
Oh, for the bonus, it goes: Chicken, Fish, Human, Dolphin, Alien, and Cat
*Warning: NSFW? Contains fetus pee pees and hoo hoos. Dead. Serious.
**DISREGARD THAT: I was completely wrong here. The two chromosome XY system is only used by most mammals. Those wily platypuses, fruit flies, and some trees use a 5 chromosomal XY sex-determination system. Meanwhile, for most ectothermic animals such as snakes, lizards, and turtles, sex is determined by environmental influences such as incubation temperature. Fruit flies and platypuses I was not aware of, but alligator sex determination I totally knew about. No excuse there.
Also, the silly Jurassic Park-inspired statement embarrassingly presumes that the definition of being female is merely a lack of penis. Apologies all around.