Tag Archives: evolution

The Trouble with (and without) Fish

December 8, 2013 By in Pregnancy, Science Tags: , , , 1 Comment

375664720_80723a9428_b

This week I’m posting a piece from my archives (August, 2011) that I’ve updated a little. Two things brought this post to mind: 1) the recent EPA report that women have become better informed about mercury and are making better choices at the fish counter and 2) remarkable updates from my scientist friend who is blogging her way through the world’s oceans as she collects water samples to catalog mercury levels around the globe. Both demonstrate that we are making some progress in studying and alerting people to the mercury in our waters and our fish. NB: when I say “now that I’m pregnant,” it’s 2011 me talking.

_________

Once upon a time in a vast ocean, life evolved. And then, over many millions of years, neurons and spinal cords and eyes developed, nourished all the while in a gentle bath of nutrients and algae.

Our brains and eyes are distant descendants of those early nervous systems formed in the sea. And even though our ancestors eventually sprouted legs and waddled out of the ocean, the neural circuitry of modern humans is still dependent on certain nutrients that their water-logged predecessors had in abundance.

This obscure fact about a distant evolution has recently turned into a major annoyance for me now that I’m pregnant. In fact, whether they know it or not, all pregnant women are trapped in a no-win dilemma over what they put into their stomachs. Take, for instance, a popular guidebook for pregnant women. On one page, it advocates eating lots of seafood while pregnant, explaining that fish contain key nutrients that the developing eyes and brain of the fetus will need. A few pages later, however, the author warns that seafood contains methylmercury, a neurotoxic pollutant, and that fish intake should be strictly curtailed. What is a well-meaning pregnant lady to do?

On a visceral level, nothing sounds worse than poisoning your child, so many women reduce their seafood intake while pregnant. I have spoken with women who cut all seafood out of their diet while pregnant, for fear that a little exposure could prove to be too much. They had good reason to be worried. Extreme methylmercury poisoning episodes in Japan and Iraq in past decades have shown that excessive methylmercury intake during pregnancy can cause developmental delays, deafness, blindness, and seizures in the babies exposed.

But what happens if pregnant women eliminate seafood from their diet altogether? Without careful supplementation of vital nutrients found in marine ecosystems, children face neural setbacks or developmental delays on a massive scale. Consider deficiencies in iodine, a key nutrient readily found in seafood. Its scarcity in the modern land-based diet was causing mental retardation in children – and sparked the creation of iodized salt (salt supplemented with iodine) to ensure that the nutritional need was met.

4566018341_8bc00c8aef_o

Perhaps the hardest nutrient to get without seafood is an omega-3 fatty acid known as DHA. In recent years, scientists have learned that this particular fatty acid is essential for proper brain development and functioning, yet it is almost impossible to get from non-aquatic dietary sources. At the grocery store, you’ll find vegetarian products that claim to fill those needs by supplying the biochemical precursor to DHA (found in flaxseed, walnuts, and soybean oils), but it’s not clear that the precursor will do the trick. Our bodies take a while to synthesize DHA from its precursor. In fact, we may burn much of the precursor for energy before we manage to convert it to DHA.

The best way for pregnant women to meet the needs of their growing babies is to eat food from marine sources. Yet thanks to global practices of burning coal and disposing of industrial and medical waste, any seafood women eat will expose their offspring to some amount of methylmercury. There’s no simple solution to this problem, although studies suggest that child outcomes are best when women consume ample seafood while avoiding species with higher levels of methylmercury (such as shark, tilefish, walleye, pike, and some types of tuna). It also matters where the fish was caught. Mercury levels will be higher in fish from mercury-polluted waters – one of the reasons that it’s important to catalog mercury levels around the globe.

Unless we start cleaning up our oceans, pregnant women will continue to face this awful decision each time they sit down at the dinner table. Far worse, we may face future generations with lower IQs and developmental delays regardless of which choice their mothers make. Thanks to shoddy environmental oversight, we may be saddling our children with brains that don’t work as well as our own. And that is something I truly can’t swallow.

____

Photo credits:

Photo 1: by Gideon (malias) on Flickr, used via Creative Commons license

Photo 2: by @Doug88888 on Flickr, used via Creative Commons license

Locked Away

August 7, 2011 By in Books, Genes, Pregnancy Tags: , , , , , 3 Comments

The results are in. The ultrasound was conclusive. And despite my previously described hunch that our growing baby is a boy, she turned out to be a girl. We are, of course, ecstatic. A healthy baby and a girl to boot! As everyone tells us, girls are simply more fun.

As I was reading in my pregnancy book the other day, I came across an interesting bit of trivia about baby girls. At this point in my pregnancy (nearly 6 months in), our baby’s ovaries contain all the eggs she’ll have for her entire life. As I mentioned in a prior post, the fact that a female fetus develops her lifetime supply of eggs in utero represents a remarkable transgenerational link. In essence, half of the genetic material that makes up my growing baby already existed inside my mother when she was pregnant. And now, inside me, exists half of the genetic material that will become all of the grandchildren I will ever have. This is the kind of link that seems to mix science and spirituality, that reminds us that, though we are a mere cluster of cells, there’s a poetry to the language of biology and Life.

But after stumbling upon this factoid about our baby’s eggs, I was also struck by a sense that somewhere someone seemed to have his or her priorities mixed up. If our baby were born today, she would have a slim chance of surviving. Her intestines, cerebral blood vessels, and retinas are immature and not ready for life outside the womb. Worse still, the only shot her lungs would have at functioning is with the aid of extreme medical intervention. The order of it all seems crazy. My baby is equipped with everything she’ll need to reproduce decades in the future, yet she lacks the lung development to make it five minutes in the outside world. What was biology thinking?

Then I remembered two delightful popular science books I’d read recently, The Red Queen by Matt Ridley and Life Ascending by Nick Lane. Both described the Red Queen Hypothesis of the evolution of sex, which states that the reason so much of the animal kingdom reproduces sexually (rather than just making clones of itself) is to ‘outwit’ parasites. In short, if each generation of humans were the same as the next, parasites large and microbial could evolve to overtake us. By mixing up our genetic makeup through sexual reproduction, we make it harder for illnesses to wipe us out. Like the Red Queen from Lewis Carroll’s classic, we keep running in order to stay in the same place (which is one step ahead of parasites and disease).

Just as there are parasitic organisms and bacteria, one might say that there are parasitic genes. For example, mutations in the DNA of our own replicating cells can cause cancer, which is essentially a self-made, genetic parasite. Moreover, retroviruses like HIV are essentially bits of genetic material that invade our bodies and can insert themselves into the DNA of our cells. And the ultimate road to immortality for a parasitic gene would be to hitch a ride on the back of reproduction. Imagine what an easy life that would be! If a retrovirus could invade the eggs in the ovaries, it would be passed on from one generation to the next without doing one iota of work. It’s the holy grail of parasitic invasion – get thee to the ovaries! According to Matt Ridley in another of his books, The Origins of Virtue, the human germ line is segregated from the rest of the growing embryo by 56 days after fertilization. Within two months of conception, the cells that will give rise to all of the embryo’s eggs (or sperm, in males) are already cordoned off. They are kept safe until they are needed many years in the future.

So perhaps my little baby’s development isn’t as backwards as it seemed at first. Yes, lungs are important. But when you’ve got something of value to others, it makes practical sense to hurry up and lock it away.

The Trouble with (and without) Fish

August 2, 2011 By in Pregnancy, Science Tags: , , , 1 Comment

Once upon a time in a vast ocean, life evolved. And then, over many millions of years, neurons and spinal cords and eyes developed, nourished all the while in a gentle bath of nutrients and algae.

Our brains and eyes are distant descendants of those early nervous systems formed in the sea. And even though our ancestors eventually sprouted legs and waddled out of the ocean, the neural circuitry of modern humans is still dependent on certain nutrients that their water-logged predecessors had in abundance.

This obscure fact about a distant evolution has recently turned into a major annoyance for me now that I’m pregnant. In fact, whether they know it or not, all pregnant women are trapped in a no-win dilemma over what they put into their stomachs. Take, for instance, a popular guidebook for pregnant women. On one page, it advocates eating lots of seafood while pregnant, explaining that fish contain key nutrients that the developing eyes and brains of the fetus will need. A few pages later, however, the author warns that seafood contains methylmercury, a neurotoxic pollutant, and that fish intake should be strictly curtailed. What is a well-meaning pregnant lady to do?

On a visceral level, nothing sounds worse than poisoning your child with mercury, and so many women reduce their seafood intake while pregnant. I have spoken with women who cut all seafood out of their diet while pregnant, for fear that a little exposure could prove to be too much. They had good reason to be worried. Extreme methylmercury poisoning episodes in Japan and Iraq in past decades have shown that excessive methylmercury intake during pregnancy can cause developmental delays, deafness, blindness, and seizures in the babies exposed.

But what happens if pregnant women eliminate seafood from their diet altogether? Without careful supplementation of vital nutrients found in marine ecosystems, children face neural setbacks or developmental delays on a massive scale. Consider deficiencies in iodine, a key nutrient readily found in seafood. Its scarcity in the modern land-based diet was causing mental retardation in children – and sparked the creation of iodized salt (salt supplemented with iodine) to ensure that the nutritional need was met.

Perhaps the hardest nutrient to get without seafood is an omega-3 fatty acid known as DHA. In recent years, scientists have learned that this particular fatty acid is essential for proper brain development and functioning, yet it is almost impossible to get from non-aquatic dietary sources. At the grocery store, you’ll find vegetarian products that claim to fill those needs by supplying the biochemical precursor to DHA (found in flaxseed, walnuts, and soybean oils), but we now know that the precursor simply won’t cut it. Our bodies are remarkably slow at synthesizing DHA from its precursor. In fact, we burn the vast majority of the precursor for energy before we have the chance to convert it to DHA.

So pregnant women must eat food from marine sources if they are to meet all the needs of their growing babies. Yet thanks to global practices of burning coal and disposing of industrial and medical waste, any seafood women eat will expose their offspring to some amount of methylmercury. There’s no simple solution to this problem, although recent studies suggest that child outcomes are best when women consume ample seafood while avoiding species with higher levels of methylmercury (such as shark, tilefish, walleye, pike, and some types of tuna). Of course much is still unknown. Exactly how much DHA intake is enough? And since mercury levels vary based on where the fish was caught and what waste was released nearby, you can never be sure it’s safe to eat.

Unless we start cleaning up our oceans, pregnant women will continue to face this awful decision each time they sit down at the dinner table. Far worse, we may face future generations with lower IQs and developmental delays regardless of which choice their mothers make. Thanks to shoddy environmental oversight, we may be saddling our children with brains that don’t work as well as our own. And that is something I truly can’t swallow.

How the Giraffe Didn’t Get His Long Neck

January 30, 2011 By in Genes Tags: , , , , , 1 Comment

iStock_000009818096XSmallIt’s the early 19th century, before Darwin’s Origin of Species. Before Mendel’s peas and Watson and Crick’s double helix. Scientists are struggling with the big questions of inheritance and reproduction without the aid of modern scientific methods. In this vacuum of concrete information, odd theories gained traction – some based on racial or social agendas, others on intuition or supposition.

Lamarckism, or soft inheritance, was one of the more pervasive of these ideas. According to the theory, organisms can inherit acquired traits. In the days before Darwin’s evolutionary theory, Lamarckism helped explain why organisms were so well adapted to their environments. Take the example of the giraffe’s long neck. A giraffe of yore (when giraffes had shorter necks) had to stretch its neck to reach the luscious leaves further up on tree branches. All that stretching lengthened its neck a little, and this longer neck was passed on to its offspring, who in turn stretched their necks and sired offspring who could reach even higher and munch the choicest leaves. It went on like this until giraffes were tall enough that they didn’t have to strain to reach leaves anymore.

It was a neat explanation that appealed to many 19th century scientists; even Darwin occasionally made use of it. But the theory had a nasty side as well. People applied it to humans and used it to explain differences between races or socioeconomic classes, calling the phenomenon degeneration. The mental and physical effects of years spent boozing and behaving badly would be passed down from father to son to grandson, each successively worse than his predecessor as the collective sum of each reckless lifetime added up. There was a technical term for the poor souls who wound up literally inheriting the sins of their fathers: degenerates. Certain scientists (or pseudoscientists) of the era, such as Benedict Morel and Cesare Lombroso, used the ideas of soft inheritance and degeneration to explain how violence, poverty, and criminality were heritable and could be categorized and studied.

Lamarckism, in the hands of Morel and others, offered a credible explanation of why the son of an alcoholic was more likely to be an alcoholic himself. But it did so by implying that the poor, the miserable, the suffering were inherently inferior to those with better, healthier (and probably wealthier) lifestyles. The poor were genetically degenerate, and they had no one to blame but themselves.

Thank god, thank god, Lamarckism and its corollary, degeneration, were debunked. By the 20th century, scientists knew that inheritance didn’t work that way. Our genetic information isn’t changed by what we do during our lifetimes. Besides, our sex cells are segregated from the other cells in our bodies. We don’t descend from our mothers, subject to all the stresses, strains, and yes, even boozing that their brains and bodies may have experienced. Instead, we descend from their ovaries. And thankfully, those things are well protected.

Only there’s a catch. In the last few decades, we’ve learned that while Lamarckism isn’t correct, it isn’t entirely wrong either. We’ve learned this through the field of epigenetics (literally, above genetics). This burgeoning field has helped us understand why the causes of so many heritable diseases still elude us, nearly a decade after we sequenced the human genome. Epigenetics adds untold complexity to an already complex genome. Some of its mechanisms are transient, others last a lifetime, but they all regulate gene expression and are necessary for normal growth and development. Thanks to them, females inactivate one of their X chromosomes (so women don’t get a double dose of proteins from that set of genes). Epigenetic mechanisms also oversee cellular differentiation, the process by which embryonic cells containing identical genetic material become skin cells, hepatocytes, neurons, and every other diverse cell type in the human body.

It now appears that epigenetic factors play an enormous role in human health. And what we do in our lives, the choices we make, affect our epigenome. Exposure to chemicals, stressors, or dietary changes can cause long-lasting tags to sit on our DNA or chromatin, controlling which genes are read and transcribed into proteins. For example, chronic cocaine use causes lasting epigenetic changes in the nucleus accumbens, a brain area linked to addiction. These changes boost plasticity and drug-related gene expression, which in turn probably contribute to the reinforcing effects of the drug.

But that’s not all. Epigenetic effects can span generations. No, the hardships of your parents’ lifetimes aren’t literally passed on to you in a cumulative fashion, giving you that longer neck or boozier disposition that Lamarckism might predict. Nonetheless, what your parents (and even your grandmother) did before you were born can be affecting your epigenome today.

It’s pretty wild stuff. Even if you’ve never met your maternal grandmother, even if she died long before your birth, her experiences and behavior could be affecting your health. First of all, the prenatal environment your mother experienced can have epigenetic effects on her that then propagate on to the next generation (you). Moreover, all the eggs a female will ever make have already formed in her ovaries by the time she’s born. They may not be mature, but they are there, DNA and all. I think that’s a pretty amazing transgenerational link. It means that half the strands of DNA that wound up becoming you were initially made inside your grandmother’s body. As science reveals the power of the prenatal environment, evidence is mounting that even what your grandmother ate during your mother’s gestational period and whether she suffered hardships like famine can alter your own risk for heart disease or diabetes.

Luckily, epigenetic gene regulation is softer and less absolute than its cousin Lamarckism. It is reversible and it can’t accumulate, generation upon generation, to create a degenerate class. The science of today is more humane than the old guys predicted, but it doesn’t let us off the hook. Epigenetics should remind us that we must be thoughtful in how we live. Our choices matter, for ourselves and for our offspring. We don’t yet understand how epigenetic mechanisms control our health and longevity, but that isn’t stopping our bodies from making us pay for what we do now.

%d bloggers like this: