I’m a medical doctor, but I’m also an anatomist, with a long-standing interest in embryology, or the study of how embryos develop. It’s a fascinating area to research, and I love teaching about it as well.
But it wasn't until the birth of my first baby, in 2010, that the subject suddenly came alive for me — in a very real way.
Becoming a mother transformed the way I look at the world. I had an incredible, almost mystical, feeling of being connected — connected with my own ancestors, and connected with my descendants. I felt like more than an individual: I was a link in the chain of life. I was giving birth to a daughter, just as I had come from my mother, and she from her mother, and back and back in time.
There’s so much about childbirth that really is momentous. You are deeply, intimately involved with what can only be described as a biological miracle: You have made a baby!
In our 21st-century world we have many ways of understanding what’s going on during pregnancy, beyond our own senses and intuition. We have much more information available to us than our parents and grandparents had.
For example, at around 12 weeks after conception, an ultrasound scan reveals the fetus. So much has already been formed by this time: The tiny fetus already has arms and legs, fingers and toes, guts and a beating heart. It already looks like a miniature baby.
That ultrasound image can be reassuring, and I felt soaring joy when I saw my baby for the first time on the screen.
But the first real, visceral awareness of the baby growing inside me only happened when I felt her, a good two months after I had seen her on the scan. I was suddenly in touch with the baby inside me, and from then on, I felt her moving every day.
What I Wish More People Knew About the Amazing Process of Human Development
Even though it was obvious that this baby would soon appear on the outside, it still felt like a physical impossibility to me.
I don’t just mean the feat of getting a human baby out. It was the idea that this fetus-on-the-inside would become a baby-on-the-outside that seemed inconceivable. And this is me: a trained doctor, an anatomist, with years of teaching embryology and reproductive anatomy behind me!
However much I knew about embryonic development, I still felt that what was happening inside me was utterly miraculous.
So I decided to write a book about it. I wanted to capture the wonder of human development that I knew existed. I thought it a great shame that you only got to hear this amazing story in all its fabulous detail if you go to med school or study anatomy.
This is a story that is relevant to absolutely everyone. After all, we were all babies, fetuses, embryos once.
Each one of us has gone on an amazing journey, passing through stages of development: beginning with that single, fertilized egg, and then developing and growing to look like a mulberry, a flattened disc, a limbless worm, and a creature that should end up with gills.
It’s such a stunning process of transformation, as incredible as the metamorphosis of a caterpillar into a butterfly.
Here are just a few of the amazing stages of a developing embryo that I wish more people knew about:
After Fertilization: Your Cells Multiply
Just three days after fertilization, you’ve grown from a single cell into a compact ball of 16 cells, known as a morula, which comes from the Latin for “mulberry,” which it somewhat resembles.
Already, cells on the outside and on the inside of the ball are destined for different fates: Outer cells will form part of the placenta; inner cells will make you.
Week 3 of Development: You're a Flat Disc
At this stage, you’re a flat disc, looking something like a flattened pear. Running down the middle of you is something called the notochord: a rod of cells that later gets obliterated when your spine forms.
The presence of a notochord in the human embryo marks us out as "chordates," meaning we belong to a group of animals that all possess this feature, including all mammals, amphibians, reptiles, birds, and fish.
Week 4 of Development: Your Heart Begins to Beat
Although your heart starts to beat at this stage, it doesn’t look much like an adult heart yet — more like a fish’s heart. It's an echo of an ancient, fishy ancestry!
In the fourth week of development, the flat disc that was once you has rolled up into a cylinder. In fact, you’re more like a series of nested cylinders, stacked inside each other.
The innermost one is your gut tube. It’s blind-ended at the moment but will eventually open at both ends to form your mouth at one end and your anus at the other.
Your lungs sprout off your gut tube. It makes sense if you think about it — your airway still opens into a part of your gut tube: The uppermost part of your airway, the larynx, opens into the pharynx, which becomes the esophagus lower down in the neck.
Week 5 of Development: You Start to Grow Limbs
Here, there are more weird similarities with fish. There are ridges in the embryo’s neck, which look as though they might become gills. They never do, of course, but once again they’re a reminder that your ancestors were fish.
Evolution loves to recycle, and the cartilage, blood vessels, and muscles that were all part of the gills in your fish ancestors are now repurposed to become bits of your larynx, neck muscles, and carotid arteries.
In that fifth week of development, you also start to grow limbs. These start off as tiny buds, then grow longer. Hand and foot “plates” develop at the ends of the limbs. By eight weeks, cell death in these plates carves out separate fingers and toes.
Week 6 of Development: Your Genitals Start to Develop
During this stage, the developing embryo has the beginnings of external genitals.
But it's a unisex “kit of parts” that will later get transformed into a penis and scrotum in a male baby, and clitoris and labia in a female baby.
Adapted from my new book, The Incredible Unlikeliness of Being: Evolution and the Making of Us, available now.
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