When I was approximately nine weeks pregnant with my first child,

I found out I'm a carrier for a fatal genetic disorder

called Tay-Sachs disease.

What this means

is that one of the two copies of chromosome number 15

that I have in each of my cells

has a genetic mutation.

Because I still have one normal copy of this gene,

the mutation doesn't affect me.

But if a baby inherits this mutation from both parents,

if both copies of this particular gene don't function properly,

it results in Tay-Sachs,

an incurable disease

that progressively shuts down the central nervous system

and causes death by age five.

For many pregnant women, this news might produce a full-on panic.

But I knew something that helped keep me calm

when I heard this bombshell about my own biology.

I knew that my husband,

whose ancestry isn't Eastern European Jewish like mine,

had a very low likelihood

of also being a carrier for the Tay-Sachs mutation.

While the frequency of heterozygotes,

individuals who have one normal copy of the gene

and one mutated copy,

is about one out of 27 people among Jews of Ashkenazi descent, like me,

in most populations,

only one in about 300 people carry the Tay-Sachs mutation.

Thankfully, it turned out I was right not to worry too much.

My husband isn't a carrier,

and we now have two beautiful and healthy children.

As I said,

because of my Jewish background,

I was aware of the unusually high rate of Tay-Sachs in the Ashkenazi population.

But it wasn't until a few years after my daughter was born

when I created and taught a seminar in evolutionary medicine at Harvard,

that I thought to ask,

and discovered a possible answer to,

the question "why?"

The process of evolution by natural selection

typically eliminates harmful mutations.

So how did this defective gene persist at all?

And why is it found at such a high frequency

within this particular population?

The perspective of evolutionary medicine offers valuable insight,

because it examines how and why

humans' evolutionary past has left our bodies vulnerable

to diseases and other problems today.

In doing so,

it demonstrates that natural selection doesn't always make our bodies better.

It can't necessarily.

But as I hope to illustrate with my own story,

understanding the implications of your evolutionary past

can help enrich your personal health.

When I started investigating Tay-Sachs using an evolutionary perspective,

I came across an intriguing hypothesis.

The unusually high rate of the Tay-Sachs mutation

in Ashkenazi Jews today

may relate to advantages the mutation gave this population

in the past.

Now I'm sure some of you are thinking,

"I'm sorry, did you just suggest that this disease-causing mutation

had beneficial effects?"

Yeah, I did.

Certainly not for individuals who inherited two copies of the mutation

and had Tay-Sachs.

But under certain circumstances,

people like me,

who had only one faulty gene copy,

may have been more likely to survive, reproduce

and pass on their genetic material,

including that mutated gene.

This idea that there can be circumstances in which heterozygotes are better off

might sound familiar to some of you.

Evolutionary biologists call this phenomenon

heterozygote advantage.

And it explains, for example,

why carriers of sickle cell anemia

are more common among some African and Asian populations

or those with ancestry from these tropical regions.

In these geographic regions, malaria poses significant risks to health.

The parasite that causes malaria, though,

can only complete its life cycle in normal, round red blood cells.

By changing the shape of a person's red blood cells,

the sickle cell mutation confers protection against malaria.

People with the mutation aren't less likely to get bitten

by the mosquitoes that transmit the disease,

but they are less likely to get sick or die as a result.

Being a carrier for sickle cell anemia

is therefore the best possible genetic option

in a malarial environment.

Carriers are less susceptible to malaria,

because they make some sickled red blood cells,

but they make enough normal red blood cells

that they aren't negatively affected by sickle cell anemia.

Now in my case,

the defective gene I carry won't protect me against malaria.

But the unusual prevalence of the Tay-Sachs mutation

in Ashkenazi populations

may be another example of heterozygote advantage.

In this case, increasing resistance to tuberculosis.

The first hint of a possible relationship between Tay-Sachs and tuberculosis

came in the 1970s,

when researchers published data

showing that among the Eastern European-born grandparents

of a sample of American Ashkenazi children born with Tay-Sachs,

tuberculosis was an exceedingly rare cause of death.

In fact, only one out of these 306 grandparents

had died of TB,

despite the fact that in the early 20th century,

TB caused up to 20 percent of deaths in large Eastern European cities.

Now on the one hand, these results weren't surprising.

People had already recognized

that while Jews and non-Jews in Europe

had been equally likely to contract TB during this time,

the death rate among non-Jews was twice as high.

But the hypothesis that these Ashkenazi grandparents

had been less likely to die of TB

specifically because at least some of them were Tay-Sachs carriers

was novel and compelling.

The data hinted

that the persistence of the Tay-Sachs mutation

among Ashkenazi Jews

might be explained by the benefits of being a carrier

in an environment where tuberculosis was prevalent.

You'll notice, though,

that this explanation only fills in part of the puzzle.

Even if the Tay-Sachs mutation persisted

because carriers were more likely to survive,

reproduce and pass on their genetic material,

why did this resistance mechanism proliferate

among the Ashkenazi population in particular?

One possibility is that the genes and health of Eastern European Jews

were affected not simply by geography

but also by historical and cultural factors.

At various points in history

this population was forced to live in crowded urban ghettos

with poor sanitation.

Ideal conditions for the tuberculosis bacterium to thrive.

In these environments, where TB posed an especially high threat,

those individuals who were not carriers of any genetic protection

would have been more likely to die.

This winnowing effect

together with a strong cultural predilection

for marrying and reproducing only within the Ashkenazi community,

would have amplified the relative frequency of carriers,

boosting TB resistance

but increasing the incidence of Tay-Sachs as an unfortunate side effect.

Studies from the 1980s support this idea.

The segment of the American Jewish population

that had the highest frequency of Tay-Sachs carriers

traced their descent

to those European countries where the incidence of TB was highest.

The benefits of being a Tay-Sachs carrier were highest

in those places where the risk of death due to TB was greatest.

And while it was unclear in the 1970s or '80s

how exactly the Tay-Sachs mutation offered protection against TB,

recent work has identified

how the mutation increases cellular defenses against the bacterium.

So heterozygote advantage can help explain

why problematic versions of genes persist at high frequencies

in certain populations.

But this is only one of the contributions evolutionary medicine can make

in helping us understand human health.

As I mentioned earlier,

this field challenges the notion

that our bodies should have gotten better over time.

An idea that often stems from a misconception

of how evolution works.

In a nutshell,

there are three basic reasons why human bodies,

including yours and mine,

remain vulnerable to diseases and other health problems today.

Natural selection acts slowly,

there are limitations to the changes it can make

and it optimizes for reproductive success,

not health.

The way the pace of natural selection affects human health

is probably most obvious

in people's relationship with infectious pathogens.

We're in a constant arms race with bacteria and viruses.

Our immune system is continuously evolving to limit their ability to infect,

and they are continuously developing ways to outmaneuver our defenses.

And our species is at a distinct disadvantage

due to our long lives and slow reproduction.

In the time it takes us to evolve one mechanism of resistance,

a pathogenic species will go through millions of generations,

giving it ample time to evolve,

so it can continue using our bodies as a host.

Now what does it mean that there are limitations

to the changes natural selection can make?

Again, my examples of heterozygote advantage

offer a useful illustration.

In terms of resisting TB and malaria,

the physiological effects of the Tay-Sachs and sickle cell anemia mutations

are good.

Taken to their extremes, though,

they cause significant problems.

This delicate balance highlights the constraints

inherent in the human body,

and the fact that the evolutionary process

must work with the materials already available.

In many instances,

a change that improves survival or reproduction

in one sense

may have cascading effects that carry their own risk.

Evolution isn't an engineer that starts from scratch

to create optimal solutions to individual problems.

Evolution is all about compromise.

It's also important to remember,

when considering our bodies' vulnerabilities,

that from an evolutionary perspective,

health isn't the most important currency.

Reproduction is.

Success is measured not by how healthy an individual is,

or by how long she lives,

but by how many copies of her genes she passes to the next generation.

This explains why a mutation

like the one that causes Huntington's disease,

another degenerative neurological disorder,

hasn't been eliminated by natural selection.

The mutation's detrimental effects

usually don't appear until after the typical age of reproduction,

when affected individuals have already passed on their genes.

As a whole,

the biomedical community focuses on proximate explanations

and uses them to shape treatment approaches.

Proximate explanations for health conditions

consider the immediate factors:

What's going on inside someone's body right now

that caused a particular problem.

Nearsightedness, for example,

is usually the result of changes to the shape of the eye

and can be easily corrected with glasses.

But as with the genetic conditions I've discussed,

a proximate explanation only provides part of the bigger picture.

Adopting an evolutionary perspective

to consider the broader question of why do we have this problem

to begin with --

what evolutionary medicine calls the ultimate perspective --

can give us insight into nonimmediate factors

that affect our health.

This is crucial,

because it can suggest ways by which you can mitigate your own risk

or that of friends and family.

In the case of nearsightedness,

some research suggests

that one reason it's becoming more common in some populations

is that many people today,

including most of us in this room,

spend far more time reading, writing

and engaging with various types of screen