Evolution of Human Pregnancy
Human Pregnancy Sucks. But WHY?
When we compare humans to other animals, humans have a more challenging birth, even when compared to the birth of our closest ancestors.
Some animals have it easy…
Opossums have pregnancies that last only about 2 weeks! However, they are marsupials, which means they give birth to offspring that are basically helpless. Marsupials carry their young inside their pouch for two to three months.
Opossums have pregnancies that last only about 2 weeks! However, they are marsupials, which means they give birth to offspring that are basically helpless. Marsupials carry their young inside their pouch for two to three months.
If a 140-pound mother gives birth to a 7-pound infant, then that infant represents about 5 percent of her body weight.
On the other hand, when a 1,000-pound horse gives birth to a 100-pound foal, that foal represents 10 percent of her body weight, or twice as much.
In pigs, a full-size sow might weigh 600 to 800 pounds, but her piglets might weigh only about 1 pound, or less than 1 percent of her weight.
Delayed Pregnancy!
Nine-banded armadillos, bears and badgers armadillos practice delayed implantation.
They can hold an embryo in a dormant state until conditions are good for a pregnancy.
Imagine holding your embryo in until you finished your college education! COOL!
Lucky Lizard!
When a lizard gives birth, its offspring can already walk and hunt.
On the other hand, when a 1,000-pound horse gives birth to a 100-pound foal, that foal represents 10 percent of her body weight, or twice as much.
In pigs, a full-size sow might weigh 600 to 800 pounds, but her piglets might weigh only about 1 pound, or less than 1 percent of her weight.
Delayed Pregnancy!
Nine-banded armadillos, bears and badgers armadillos practice delayed implantation.
They can hold an embryo in a dormant state until conditions are good for a pregnancy.
Imagine holding your embryo in until you finished your college education! COOL!
Lucky Lizard!
When a lizard gives birth, its offspring can already walk and hunt.
What is to blame?
The difficulties that come with human birth stems from a combination of
long labor times
big infant heads
narrow maternal hips
the size of the fetus relative to the mother
The difficulties that come with human birth stems from a combination of
long labor times
big infant heads
narrow maternal hips
the size of the fetus relative to the mother
Some animals have a harder time than the pig or the ‘possum…
Imagine a 230 Pound Baby?
At up to 23 months, elephants boast the longest gestation period of any land animal. They also boast big babies: a newborn elephant weighs about 230 pounds (105 kilograms).
Imagine being pregnant 3.5 YEARS!
The elephant's got nothing on the frilled shark, however. These primitive, eel-like animals have extraordinarily slow-growing embryos, which put on only about a half-inch (1.4 cm) each month. At that rate, the total pregnancy length of a frilled shark is estimated at three and a half years.
- Male Pregnancy!
Male seahorses are famously active dads, even taking on the role of egg incubator.
Females deposit unfertilized eggs into a pouch on the male's belly.
He fertilizes and protects these eggs for several weeks.
The male then gives birth to around 200 hatchlings.
Then it's bon voyage to the small fry, as seahorses do not nurture their young.
Getting’ Busy!
- Constant Pregnancy
This famous classroom pet is a champion breeder.
Guinea pigs are capable of getting pregnant at as early as 4 weeks of age.
Females go into heat only two to 15 hours after giving birth to a litter
The Evolution of Human Birth: An Incredible Story a Million Years in the Making
human childbirth evolved and paved the way for large brains.
Lucy
Lucy, an Australopithecus afarensis, is one of the first known relatives of mankind and her species body’s structure was designed for walking upright.
Lucy is 3.2 million years old
Australiopethicus - Lucy
Her pelvis is similar to modern human females.
The Sideways Tilt
A Hominin Infant Would Tilt Only a Tiny Bit Compared to a Human Baby
By examining their models, the scientists noticed that, just like it happens with humans, an infant A. afarensis probably entered the birth canal sideways.
The Sideways Tilt
Theoretically, however, a baby A. afarensis would not have to tilt too much to make way for its shoulders as its head slid down the birth canal, like for example a human baby’s head has to rotate 90 degrees during childbirth. DeSilva explained to Live Science:
Baby Head Versus Birth Canal
The brain of a newborn chimp is less than half the size of a newborn human baby.
UNFAIR! - the overall size of the birth canal is about the same as in humans.
For chimpanzees and other primates, childbirth is relatively easy.
The brain of a newborn chimp is less than half that of a newborn human baby, although the overall size of the birth canal is about the same as in humans.
A child’s head is large in proportion to their body size
Humans have large brains
Babies are born before their brains have fully developed. That’s by design.
Trying to pass an adult-sized head during delivery is a frightening proposition.
Cost of Big Brain!
the brains — and skulls — of developing babies can't grow bigger than they do in the womb than they already do, because they wouldn't fit through the mother's pelvis.
Human Body Adaptations to Giving Birth
1. Larger Pelvis
2. “soft spots”
Fortunately, an infant's skull can change shape as it squeezes through because its cranial bones don't entirely fuse together for at least 2 years after birth.
Soft spots allow the brain to grow.
Thanks to those unfused skull bones, the baby's head can not only be molded to fit through the pelvic canal, but can also accommodate the explosive brain growth that takes place after birth:
In the first few years of life, the human brain doubles in size from about 400 cc to 800 cc, and ultimately reaches an adult average of about 1400 cc.
Limits due to brain size and walking upright
While the human pelvis has widened considerably over the course of evolution, the demands of upright walking have put constraints on how wide it can be.
Bipedalism has also led to a marked vertical shortening of the pelvis, leading to what researchers call the "obstetric dilemma"—the difficult tradeoff between the demands of bipedalism and having babies.
Human Newborns are Relatively Helpless
human infants are especially helpless because their brains are underdeveloped.
a human fetus would have to undergo a gestation period of 18 to 21 months instead of the usual 9 to be born at a neurological and cognitive development stage comparable to that of a chimpanzee newborn.
Big-Headed Babies!
The explanation for our nine-month gestation period and helpless newborns is that natural selection favored childbirth at an earlier stage of fetal development to accommodate for both large brain size and upright locomotion
Big-Headed Babies!
About two million years ago, our hominin ancestors began to change again.
They lost their more ape-like features such as a relatively short body, long arms and small brain. Instead they began to gain more human-like ones, like taller bodies, shorter arms and bigger brains.
That last trait in particular was bad news for female hominins.
To walk on two legs efficiently, the hominin skeleton had to be pushed and pulled into a new configuration, and that affected the pelvis.
In most primates the birth canal in the pelvis is relatively straight.
In hominins, it soon began to look very different.
Hips became relatively narrow and the birth canal became distorted – a cylinder that varied in size and shape along its length.
Big-brained adults start out life as big-brained babies.
On the one hand, female hominins had to maintain a narrow pelvis with a constricted birth canal in order to walk efficiently on two legs.
But at the same time the fetuses they carried were evolving to have larger heads, which were a tighter and tighter fit through those narrow pelvises.