Karen Rosenberg, paleoanthropologist

An expert in the evolution of childbirth peers through pelvises to glimpse the essence of our humanity. Interview by Jessica Shugart

April 02, 2013

Karen Rosenberg measures pelvis fossils to reconstruct ancient childbirth. Courtesy of the University of Delaware

Few events in our life cycle evoke as much agony, elation, and respect as childbirth. Pushing another human being through one’s birth canal is the yardstick of suffering and endurance by which all other feats are measured. In the book of Genesis, the curse of childbirth was bestowed upon women for being temptresses. But we now know that the arduous process of childbirth evolved over millions of years to culminate in the just-barely-doable miracle that thousands of women achieve each day.

Paleoanthropologist Karen Rosenberg of the University of Delaware thinks the way we give birth is one of the most defining characteristics of our humanity. Her decades of tinkering with ancient pelvis fossils has helped to uncover just how tightly our squeeze measures up to that of other creatures. While chimp babies pass straight through the birth canal with room to spare, human infants twist and turn in a precisely choreographed manner to emerge safely from the womb.

The human fossil record has revealed that the size and shape of our ancestors’ pelvises changed when they stood upright five million years ago. But if bipedalism made things tricky, Rosenberg thinks our growing brains were the ultimate birth canal sculptor. She hypothesizes that the “rotational childbirth” we experience today evolved to make way for our expanding heads.

This tortuous path through the canal couldn’t have evolved without a little help, Rosenberg says. While other animals give birth alone, humans seek out doctors, midwives, doulas, or family members in their time of need. What’s more, human babies are pathetically helpless when born, so the need for support continues long after the birthing process is done. Rosenberg’s studies of our evolutionary past have implications for the way we view childbirth and childcare today.

Rosenberg spoke about the human predicament of epic labor at a session she co-organized entitled “The Scars of Human Evolution” at the February 2013 meeting of the American Association for the Advancement of Science in Boston. SciCom’s Jessica Shugart sat down with her afterward to talk about the pangs of childbirth’s evolutionary past.

How did you first get excited about studying the evolution of human childbirth?

Childbirth is a rare time in the regular life cycle of humans where you can really see natural selection operating. It’s a very dramatic and obvious thing. I was also interested in [the evolution of human childbirth] because for most of the history of studying human evolution, attention had been on male activities like hunting. There hadn’t been as much attention to what women were doing.

Did you encounter any resistance to pursuing that type of study?

No, I wouldn’t say so. I think people were really interested, and it was actually around the time [mid 1980s] when the Lucy pelvis was first described. Some of the people who worked on that talked about birth right away.

Lucy was a member of Australopithecus afarensis, a possible human ancestor that lived roughly three million years ago, and she had a wide pelvis. Why don’t we have a nice wide pelvis like hers?

We do have a wide pelvis, but compared to our body size it’s not as wide as Lucy’s. There are locomotor constraints on how wide our pelvis can get, because it becomes very inefficient for us to walk.

But Lucy was bipedal, like us.

She was bipedal, but she didn’t have as large a pelvis in the front to back dimension as we do — she was quite flat. So it’s a tradeoff. If the pelvis gets too big, locomotion becomes inefficient. If it gets too small, you can’t give birth. The point at which we’ve settled on is dependent on our head size and shoulder size, and Lucy’s was different.

When do you think assisted birth evolved?

I’m sure that by the time of Neandertals [100,000 years ago] you have assisted birth, because by that time you have a birth canal of changing cross-sectional shape. It’s flat at the top, long in the middle, and round at the outlet. At some time, probably in Homo erectus [between 300,000 and 2 million years ago], we reached the point where you couldn’t make the pelvis wider, and it wouldn’t accommodate an increasingly large baby. Then this front-to-back expansion took place, which allowed the head to rotate. This is a hypothesis rather than something I’ve demonstrated, but I’m suggesting that the fact we’re able to give birth in this rotational way may have allowed our brains to expand.

Do you think it’s possible that we needed help with giving birth after we became bipedal, but before we developed rotational birth and bigger brains?

Once you have rotation, for sure, you benefit from assistance. But I think it’s possible that we benefited from assistance even before that. It’s possible that Lucy’s mechanism of birth was sufficiently complicated that maybe she would’ve benefited from assistance too. But I don’t know of a way to test that hypothesis.

"I would be critical of the idea that it’s somehow more natural to give birth in the water. I mean, really, we’ve been living on the land for a long time."

Do we have any evidence for assisted birth besides fossils?

No, I would say the evidence is the anatomy.

You don't have that many samples to work with, right?

The sample size is small, but it’s consistent in the morphology that it shows.

As far as infant heads...

Infant fossils are so fragmentary that they're not often discovered, or they're not recognized as human. They’re very rare, but we have discovered a couple of nice ones recently.

Has a fossil of a pregnant woman ever been discovered?

I don’t know of any in the human fossil record. There's a whale though, a very cool whale — Maiacetus from the Eocene.

How does the monkey birth mechanism compare to that in apes and humans?

They have narrow shoulders, and their pelvis is completely different. What’s similar between monkeys and humans is the relationship between the size of the baby’s head and the size of the pelvis, although theirs isn’t as extreme as ours. 

Monkeys seem to take longer [than apes] to give birth. We don’t know much outside of the lab, but if you can read the emotion of the animals, monkeys seem to be in more pain than apes.

And the monkeys come out facing the mother, unlike human babies, which are born facing the opposite direction?

Right. And monkeys are strong enough that they can pull themselves up by their mother’s fur. So you see these monkey babies actually pulling themselves up to the breast. Human babies have surprisingly strong grasps, but there’s nothing to hold onto.

Because human babies are born facing the other direction?

And because we don’t have fur.

You’ve mentioned studies showing that having a doula’s emotional support during birth reduces the length of labor and lessens the use of interventions (such as forceps delivery and C-sections). How do you explain this phenomenon?

I think that’s because we’ve evolved to give birth in that kind of environment. That’s what we expect. 

Do you think that fear of giving birth is beneficial in some way?

I don’t know if I’m willing to go that far, but I would say that the behavior we see in women as a result of their anxiety and fear leads them to do smart things. Because we’re afraid, we don’t go off into the woods and give birth alone. We say, “Oh my God, I’m having contractions, let me call my midwife and my doula and get my husband home from work.” Those things are beneficial.

What do you think about water birth?

It’s a good thing for women to have choices, and to have autonomy and control over how they give birth. Certainly there’s evidence today that women like to move around, they feel supported in the water, they like to be in the shower, all those things. But I would be critical of the idea that it’s somehow more natural to give birth in the water. I mean, really, we’ve been living on the land for a long time. 

As far as natural birth is concerned, what do you think about epidurals? Is it beneficial for women to feel what’s happening during birth?

I don’t think of epidurals or any other intervention as being different in kind than having a midwife. It’s all intervention. But I think there is good evidence that certain kinds of problems come when women have epidurals, because then they’re not active participants in the birth process. 

In the United States, more than 30 percent of babies are delivered by Caesarean section, and in some hospitals that level soars above 70 percent. Yet the World Health Organization has suggested that a 12 percent rate should be medically sufficient. How do you feel about this?

I’m critical of the high Caesarean rate, but on the other hand, you want to be above the [recommended] number rather than below that number. There are some women or babies who would die or be badly damaged if they didn’t have access to Caesarean sections, so some of them are literally lifesavers. But I don’t think it is nearly as many as we perform.

Why do you think the Caesarean rate is so high in some hospitals?

A lot of interventions are done for doctors. It’s more convenient for them; it’s simpler for them; it’s a hedge against liability. Obstetrics is the part of the medical profession where the most lawsuits happen. When there’s a problem with a baby, that’s when people go to court. That’s a lot of the reason why doctors make the decisions that they do. Another reason that has become more important recently is that women are asking for Caesarean sections because they think it will be easier [than vaginal birth].

What are the next questions you'd like to answer?

I'm starting now to work on the baby side of birth. We give birth to these very helpless infants, and we pay a huge price for that, right? Do you have children?

No, not yet.

Well, you’ll see. They just suck up every bit of energy in the house. We're paying a pretty high price for it, so maybe we're getting something from it too.

What could we get from it?

Maybe having babies that are so helpless early in their development gives them an opportunity to be exposed to stimuli at a time when it can affect the way they learn. We know that in those very early weeks of childhood, babies are already soaking up information that has to do with language and kinship and social relationships.

Might babies also do some of that learning in the womb?

We know they're hearing and learning some of those things inside the womb, but I would assume that it's done more efficiently outside of the womb.

What do you think we can learn about childbirth today from studying the evolution of human childbirth?

We should think about what women are asking for. We should stop thinking of women as just scared, emotional, anxious people and realize it makes sense for them to be looking for help. And I think we should certainly be open to the idea that the way doctors like to do things in this country is not the only way it works. 


Jessica Shugart, a graduate student in the Science Communication Program at UC Santa Cruz, earned her bachelor's degree in molecular and cell biology from UC San Diego and her PhD. in immunology from UC Berkeley. At UCSC, she has worked as a reporting intern at the Stanford University School of Medicine, the Monterey County Herald, and the San Jose Mercury News. This summer, she will report for Science News in Washington, D.C., as a science writing intern.

© 2013 Jessica Shugart