WEBVTT 00:00:01.119 --> 00:00:05.151 When I was approximately nine weeks pregnant with my first child, 00:00:05.175 --> 00:00:08.651 I found out I'm a carrier for a fatal genetic disorder 00:00:08.675 --> 00:00:10.341 called Tay-Sachs disease. 00:00:11.101 --> 00:00:12.291 What this means 00:00:12.315 --> 00:00:16.204 is that one of the two copies of chromosome number 15 00:00:16.228 --> 00:00:18.141 that I have in each of my cells 00:00:18.165 --> 00:00:19.807 has a genetic mutation. 00:00:20.458 --> 00:00:23.490 Because I still have one normal copy of this gene, 00:00:23.514 --> 00:00:25.580 the mutation doesn't affect me. 00:00:26.063 --> 00:00:29.904 But if a baby inherits this mutation from both parents, 00:00:29.928 --> 00:00:34.237 if both copies of this particular gene don't function properly, 00:00:34.261 --> 00:00:36.061 it results in Tay-Sachs, 00:00:36.085 --> 00:00:37.934 an incurable disease 00:00:37.958 --> 00:00:40.704 that progressively shuts down the central nervous system 00:00:40.728 --> 00:00:43.116 and causes death by age five. NOTE Paragraph 00:00:44.570 --> 00:00:48.800 For many pregnant women, this news might produce a full-on panic. 00:00:49.125 --> 00:00:51.478 But I knew something that helped keep me calm 00:00:51.502 --> 00:00:54.625 when I heard this bombshell about my own biology. 00:00:54.944 --> 00:00:56.365 I knew that my husband, 00:00:56.389 --> 00:01:00.087 whose ancestry isn't Eastern European Jewish like mine, 00:01:00.111 --> 00:01:01.770 had a very low likelihood 00:01:01.794 --> 00:01:05.110 of also being a carrier for the Tay-Sachs mutation. 00:01:05.569 --> 00:01:07.863 While the frequency of heterozygotes, 00:01:07.887 --> 00:01:10.656 individuals who have one normal copy of the gene 00:01:10.680 --> 00:01:12.347 and one mutated copy, 00:01:12.371 --> 00:01:18.355 is about one out of 27 people among Jews of Ashkenazi descent, like me, 00:01:18.379 --> 00:01:20.013 in most populations, 00:01:20.037 --> 00:01:24.275 only one in about 300 people carry the Tay-Sachs mutation. 00:01:25.125 --> 00:01:28.363 Thankfully, it turned out I was right not to worry too much. 00:01:28.387 --> 00:01:30.117 My husband isn't a carrier 00:01:30.141 --> 00:01:33.324 and we now have two beautiful and healthy children. 00:01:35.602 --> 00:01:36.753 As I said, 00:01:36.777 --> 00:01:38.308 because of my Jewish background, 00:01:38.332 --> 00:01:43.498 I was aware of the unusually high rate of Tay-Sachs in the Ashkenazi population. NOTE Paragraph 00:01:43.522 --> 00:01:47.141 But it wasn't until a few years after my daughter was born 00:01:47.165 --> 00:01:51.212 when I created and taught a seminar in evolutionary medicine at Harvard 00:01:51.236 --> 00:01:52.831 that I thought to ask, 00:01:52.855 --> 00:01:55.220 and discovered a possible answer to, 00:01:55.244 --> 00:01:56.894 the question "why?" 00:01:57.252 --> 00:01:59.743 The process of evolution by natural selection 00:01:59.767 --> 00:02:02.429 typically eliminates harmful mutations. 00:02:02.776 --> 00:02:06.329 So how did this defective gene persist at all? 00:02:06.353 --> 00:02:09.408 And why is it found at such a high frequency 00:02:09.432 --> 00:02:11.699 within this particular population? NOTE Paragraph 00:02:13.059 --> 00:02:17.093 The perspective of evolutionary medicine offers valuable insight, 00:02:17.117 --> 00:02:19.522 because it examines how and why 00:02:19.546 --> 00:02:23.442 humans' evolutionary past has left our bodies vulnerable 00:02:23.466 --> 00:02:26.057 to diseases and other problems today. 00:02:26.716 --> 00:02:28.057 In doing so, 00:02:28.081 --> 00:02:32.272 it demonstrates that natural selection doesn't always make our bodies better. 00:02:32.296 --> 00:02:34.025 It can't necessarily. 00:02:34.597 --> 00:02:37.394 But as I hope to illustrate with my own story, 00:02:37.418 --> 00:02:40.934 understanding the implications of your evolutionary past 00:02:40.958 --> 00:02:43.839 can help enrich your personal health. NOTE Paragraph 00:02:45.148 --> 00:02:49.076 When I started investigating Tay-Sachs using an evolutionary perspective, 00:02:49.100 --> 00:02:52.012 I came across an intriguing hypothesis. 00:02:52.401 --> 00:02:55.338 The unusually high rate of the Tay-Sachs mutation 00:02:55.362 --> 00:02:57.704 in Ashkenazi Jews today 00:02:57.728 --> 00:03:01.561 may relate to advantages the mutation gave this population 00:03:01.585 --> 00:03:02.735 in the past. 00:03:03.665 --> 00:03:05.506 Now I'm sure some of you are thinking, 00:03:05.530 --> 00:03:09.498 "I'm sorry, did you just suggest that this disease-causing mutation 00:03:09.522 --> 00:03:11.728 had beneficial effects?" 00:03:11.752 --> 00:03:13.149 Yeah, I did. 00:03:13.173 --> 00:03:16.824 Certainly not for individuals who inherited two copies of the mutation 00:03:16.848 --> 00:03:18.466 and had Tay-Sachs. 00:03:18.847 --> 00:03:20.769 But under certain circumstances, 00:03:20.793 --> 00:03:21.959 people like me, 00:03:21.983 --> 00:03:25.107 who had only one faulty gene copy, 00:03:25.131 --> 00:03:29.004 may have been more likely to survive, reproduce 00:03:29.028 --> 00:03:31.258 and pass on their genetic material. 00:03:31.282 --> 00:03:33.249 Including that mutated gene. NOTE Paragraph 00:03:34.631 --> 00:03:40.425 This idea that there can be circumstances in which heterozygotes are better off 00:03:40.449 --> 00:03:42.774 might sound familiar to some of you. 00:03:43.076 --> 00:03:45.966 Evolutionary biologists call this phenomenon 00:03:45.990 --> 00:03:48.260 heterozygote advantage. 00:03:48.284 --> 00:03:50.879 And it explains, for example, 00:03:50.903 --> 00:03:52.895 why carriers of sickle cell anemia 00:03:52.919 --> 00:03:56.942 are more common among some African and Asian populations 00:03:56.966 --> 00:04:00.101 or those with ancestry from these tropical regions. 00:04:00.768 --> 00:04:05.791 In these geographic regions, malaria poses significant risks to health. 00:04:06.546 --> 00:04:09.164 The parasite that causes malaria, though, 00:04:09.188 --> 00:04:14.688 can only complete its life cycle in normal, round red blood cells. 00:04:15.493 --> 00:04:18.548 By changing the shape of a person's red blood cells, 00:04:18.572 --> 00:04:22.865 the sickle cell mutation confers protection against malaria. 00:04:23.659 --> 00:04:26.521 People with the mutation aren't less likely to get bitten 00:04:26.545 --> 00:04:29.045 by the mosquitoes that transmit the disease, 00:04:29.069 --> 00:04:32.704 but they are less likely to get sick or die as a result. 00:04:33.450 --> 00:04:36.204 Being a carrier for sickle cell anemia 00:04:36.228 --> 00:04:39.109 is therefore the best possible genetic option 00:04:39.133 --> 00:04:40.866 in a malarial environment. 00:04:41.276 --> 00:04:43.654 Carriers are less susceptible to malaria 00:04:43.678 --> 00:04:46.640 because they make some sickled red blood cells, 00:04:46.664 --> 00:04:49.514 but they make enough normal red blood cells 00:04:49.538 --> 00:04:53.156 that they aren't negatively affected by sickle cell anemia. NOTE Paragraph 00:04:55.292 --> 00:04:56.450 Now in my case, 00:04:56.474 --> 00:05:00.450 the defective gene I carry won't protect me against malaria. 00:05:01.196 --> 00:05:04.903 But the unusually prevalence of the Tay-Sachs mutation 00:05:04.927 --> 00:05:06.799 in Ashkenazi populations 00:05:06.823 --> 00:05:10.755 may be another example of heterozygote advantage. 00:05:10.779 --> 00:05:14.311 In this case, increasing resistance to tuberculosis. 00:05:15.915 --> 00:05:20.478 The first hint of a possible relationship between Tay-Sachs and tuberculosis 00:05:20.502 --> 00:05:22.458 came in the 1970s, 00:05:22.482 --> 00:05:23.973 when researchers published data 00:05:23.997 --> 00:05:27.029 showing that among the Eastern European-born grandparents 00:05:27.053 --> 00:05:31.200 of a sample of American Ashkenazi children born with Tay-Sachs, 00:05:31.224 --> 00:05:35.018 tuberculosis was an exceedingly rare cause of death. 00:05:35.042 --> 00:05:39.153 In fact, only one out of these 306 grandparents 00:05:39.177 --> 00:05:41.019 had died of TB, 00:05:41.043 --> 00:05:44.177 despite the fact that in the early 20th century, 00:05:44.201 --> 00:05:49.021 TB caused up to 20 percent of deaths in large Eastern European cities. NOTE Paragraph 00:05:50.483 --> 00:05:53.158 Now on the one hand, these results weren't surprising. 00:05:53.182 --> 00:05:54.912 People had already recognized 00:05:54.936 --> 00:05:57.403 that while Jews and non-Jews in Europe 00:05:57.427 --> 00:06:01.149 had been equally likely to contract TB during this time, 00:06:01.173 --> 00:06:04.791 the death rate among non-Jews was twice as high. 00:06:05.657 --> 00:06:08.831 But the hypothesis that these Ashkenazi grandparents 00:06:08.855 --> 00:06:11.387 had been less likely to die of TB 00:06:11.411 --> 00:06:16.005 specifically because at least some of them were Tay-Sachs carriers 00:06:16.029 --> 00:06:17.949 was novel and compelling. 00:06:18.545 --> 00:06:19.696 The data hinted 00:06:19.720 --> 00:06:21.886 that the persistence of the Tay-Sachs mutation 00:06:21.910 --> 00:06:23.616 among Ashkenazi Jews 00:06:23.640 --> 00:06:27.237 might be explained by the benefits of being a carrier 00:06:27.261 --> 00:06:30.840 in an environment where tuberculosis was prevalent. 00:06:32.135 --> 00:06:33.491 You'll notice, though, 00:06:33.515 --> 00:06:37.213 that this explanation only fills in part of the puzzle. 00:06:37.803 --> 00:06:40.983 Even if the Tay-Sachs mutation persisted 00:06:41.007 --> 00:06:43.572 because carriers were more likely to survive, 00:06:43.596 --> 00:06:46.771 reproduce and pass on their genetic material, 00:06:46.795 --> 00:06:49.443 why did this resistance mechanism proliferate 00:06:49.467 --> 00:06:52.534 among the Ashkenazi population in particular? NOTE Paragraph 00:06:53.626 --> 00:06:58.745 One possibility is that the genes and health of Eastern European Jews 00:06:58.769 --> 00:07:01.516 were affected not simply by geography, 00:07:01.540 --> 00:07:04.905 but also by historical and cultural factors. 00:07:05.588 --> 00:07:07.302 At various points in history 00:07:07.326 --> 00:07:10.910 this population was forced to live in crowded urban ghettos 00:07:10.934 --> 00:07:12.334 with poor sanitation. 00:07:12.688 --> 00:07:16.704 Ideal conditions for the tuberculosis bacterium to thrive. 00:07:17.284 --> 00:07:21.823 In these environments, where TB posed an especially high threat, 00:07:21.847 --> 00:07:26.649 those individuals who were not carriers of any genetic protection 00:07:26.673 --> 00:07:29.006 would have been more likely to die. 00:07:29.482 --> 00:07:31.529 This winnowing effect 00:07:31.553 --> 00:07:34.522 together with a strong cultural predilection 00:07:34.546 --> 00:07:38.879 for marrying and reproducing only within the Ashkenazi community, 00:07:38.903 --> 00:07:42.625 would have amplified the relative frequency of carriers, 00:07:42.649 --> 00:07:44.609 boosting TB resistance, 00:07:44.633 --> 00:07:49.426 but increasing the incidence of Tay-Sachs as an unfortunate side effect. NOTE Paragraph 00:07:50.403 --> 00:07:53.154 Studies from the 1980s support this idea. 00:07:53.713 --> 00:07:56.334 The segment of the American Jewish population 00:07:56.358 --> 00:07:59.498 that had the highest frequency of Tay-Sachs carriers 00:07:59.522 --> 00:08:01.038 traced their descent 00:08:01.062 --> 00:08:05.098 to those European countries where the incidence of TB was highest. 00:08:05.678 --> 00:08:09.280 The benefits of being a Tay-Sachs carrier were highest 00:08:09.304 --> 00:08:13.090 in those places where the risk of death due to TB was greatest. 00:08:13.561 --> 00:08:16.743 And while it was unclear in the 1970s or '80s 00:08:16.767 --> 00:08:21.950 how exactly the Tay-Sachs mutation offered protection against TB, 00:08:21.974 --> 00:08:23.761 recent work has identified 00:08:23.785 --> 00:08:27.816 how the mutation increases cellular defenses against the bacterium. 00:08:29.315 --> 00:08:32.625 So heterozygote advantage can help explain 00:08:32.649 --> 00:08:36.395 why problematic versions of genes persist at high frequencies 00:08:36.419 --> 00:08:37.952 in certain populations. NOTE Paragraph 00:08:38.482 --> 00:08:41.933 But this is only one of the contributions evolutionary medicine can make 00:08:41.957 --> 00:08:44.106 in helping us understand human health. 00:08:44.686 --> 00:08:46.051 As I mentioned earlier, 00:08:46.075 --> 00:08:48.003 this field challenges the notion 00:08:48.027 --> 00:08:51.051 that our bodies should have gotten better over time. 00:08:51.075 --> 00:08:54.173 An idea that often stems from a misconception 00:08:54.197 --> 00:08:56.347 of how evolution works. 00:08:57.363 --> 00:08:58.537 In a nutshell, 00:08:58.561 --> 00:09:01.228 there are three basic reasons why human bodies, 00:09:01.252 --> 00:09:03.236 including yours and mine, 00:09:03.260 --> 00:09:06.942 remain vulnerable to diseases and other health problems today. 00:09:07.379 --> 00:09:09.561 Natural selection acts slowly, 00:09:09.585 --> 00:09:12.291 there are limitations to the changes it can make, 00:09:12.315 --> 00:09:15.450 and it optimizes for reproductive success, 00:09:15.474 --> 00:09:17.109 not health. NOTE Paragraph 00:09:18.641 --> 00:09:21.799 The way the pace of natural selection affects human health 00:09:21.823 --> 00:09:23.537 is probably most obvious 00:09:23.561 --> 00:09:26.934 in people's relationship with infectious pathogens. 00:09:26.958 --> 00:09:31.287 We're in a constant arms race with bacteria and viruses. 00:09:31.311 --> 00:09:36.184 Our immune system is continuously evolving to limit their ability to infect, 00:09:36.208 --> 00:09:40.776 and they are continuously developing ways to outmaneuver our defenses. 00:09:40.800 --> 00:09:43.815 And our species is at a distinct disadvantage 00:09:43.839 --> 00:09:46.839 due to our long lives and slow reproduction. 00:09:47.419 --> 00:09:52.011 In the time it takes us to evolve one mechanism of resistance, 00:09:52.035 --> 00:09:56.138 a pathogenic species will go through millions of generations, 00:09:56.162 --> 00:09:58.051 giving it ample time to evolve, 00:09:58.075 --> 00:10:00.840 so it can continue using our bodies as a host. NOTE Paragraph 00:10:02.481 --> 00:10:04.751 Now what does it mean that there are limitations 00:10:04.775 --> 00:10:07.100 to the changes natural selection can make? 00:10:07.124 --> 00:10:09.965 Again, my examples of heterozygote advantage 00:10:09.989 --> 00:10:11.856 offer a useful illustration. 00:10:12.506 --> 00:10:15.371 In terms of resisting TB and malaria, 00:10:15.395 --> 00:10:19.989 the physiological effects of the Tay-Sachs and sickle cell anemia mutations 00:10:20.013 --> 00:10:21.385 are good. 00:10:21.871 --> 00:10:23.434 Taken to their extremes, though, 00:10:23.458 --> 00:10:25.759 they cause significant problems. 00:10:26.339 --> 00:10:29.458 This delicate balance highlights the constraints 00:10:29.482 --> 00:10:31.456 inherent in the human body, 00:10:31.480 --> 00:10:34.170 and the fact that the evolutionary process 00:10:34.194 --> 00:10:36.932 must work with the materials already available. 00:10:37.298 --> 00:10:38.583 In many instances, 00:10:38.607 --> 00:10:40.910 a change that improves survival or reproduction 00:10:40.934 --> 00:10:42.101 in one sense 00:10:42.125 --> 00:10:45.449 may have cascading effects that carry their own risk. 00:10:45.990 --> 00:10:49.290 Evolution isn't an engineer that starts from scratch 00:10:49.314 --> 00:10:52.593 to create optimal solutions to individual problems. 00:10:53.077 --> 00:10:55.783 Evolution is all about compromise. NOTE Paragraph 00:10:57.188 --> 00:10:58.736 It's also important to remember, 00:10:58.760 --> 00:11:01.069 when considering our bodies' vulnerabilities, 00:11:01.093 --> 00:11:03.276 that from an evolutionary perspective, 00:11:03.300 --> 00:11:05.839 health isn't the most important currency. 00:11:05.863 --> 00:11:07.568 Reproduction is. 00:11:07.592 --> 00:11:11.727 Success is measured not by how healthy an individual is, 00:11:11.751 --> 00:11:13.601 or by how long she lives, 00:11:13.625 --> 00:11:17.570 but by how many copies of her genes she passes to the next generation. 00:11:18.331 --> 00:11:19.833 This explains why a mutation 00:11:19.857 --> 00:11:22.386 like the one that causes Huntington's disease -- 00:11:22.410 --> 00:11:25.085 another degenerative neurological disorder, 00:11:25.109 --> 00:11:28.338 hasn't been eliminated by natural selection. 00:11:28.362 --> 00:11:30.125 The mutation's detrimental effects 00:11:30.149 --> 00:11:34.561 usually don't appear until after the typical age of reproduction, 00:11:34.585 --> 00:11:38.101 when affected individuals have already passed on their genes. NOTE Paragraph 00:11:38.855 --> 00:11:40.030 As a whole, 00:11:40.054 --> 00:11:43.744 the biomedical community focuses on proximate explanations, 00:11:43.768 --> 00:11:46.529 and uses them to shape treatment approaches. 00:11:46.942 --> 00:11:49.586 Proximate explanations for health conditions 00:11:49.610 --> 00:11:51.751 consider the immediate factors: 00:11:51.775 --> 00:11:54.704 What's going on inside someone's body right now 00:11:54.728 --> 00:11:56.861 that caused a particular problem. 00:11:57.196 --> 00:11:58.934 Nearsightedness, for example, 00:11:58.958 --> 00:12:02.268 is usually the result of changes to the shape of the eye, 00:12:02.292 --> 00:12:05.025 and can be easily corrected with glasses. 00:12:06.061 --> 00:12:09.188 But as with the genetic conditions I've discussed, 00:12:09.212 --> 00:12:13.267 a proximate explanation only provides part of the bigger picture. 00:12:13.776 --> 00:12:16.323 Adopting an evolutionary perspective 00:12:16.347 --> 00:12:20.608 to consider the broader question of why do we have this problem 00:12:20.632 --> 00:12:22.212 to begin with -- 00:12:22.236 --> 00:12:26.340 what evolutionary medicine calls the ultimate perspective -- 00:12:26.364 --> 00:12:29.277 can give us insight into nonimmediate factors 00:12:29.301 --> 00:12:30.835 that affect our health. 00:12:31.158 --> 00:12:32.309 This is crucial, 00:12:32.333 --> 00:12:36.396 because it can suggest ways by which you can mitigate your own risk 00:12:36.420 --> 00:12:38.261 or that of friends and family. NOTE Paragraph 00:12:39.571 --> 00:12:41.269 In the case of nearsightedness, 00:12:41.293 --> 00:12:43.039 some research suggests 00:12:43.063 --> 00:12:46.355 that one reason it's becoming more common in some populations 00:12:46.379 --> 00:12:48.283 is that many people today, 00:12:48.307 --> 00:12:50.529 including most of us in this room, 00:12:50.553 --> 00:12:53.968 spend far more time reading, writing 00:12:53.992 --> 00:12:56.513 and engaging with various types of screen 00:12:56.537 --> 00:13:00.577 than we do outside, interacting with the world on a bigger scale. 00:13:01.331 --> 00:13:04.664 In evolutionary terms, this is a recent change. 00:13:05.061 --> 00:13:07.466 For most of human evolutionary history, 00:13:07.490 --> 00:13:10.815 people used their vision across a broader landscape, 00:13:10.839 --> 00:13:14.259 spending more time in activities like hunting and gathering. 00:13:14.839 --> 00:13:19.418 The increase in recent years in what's termed "near work," 00:13:19.442 --> 00:13:22.443 focusing intensely on objects directly in front of us 00:13:22.467 --> 00:13:24.315 for long periods of time, 00:13:24.339 --> 00:13:26.379 strains our eyes differently 00:13:26.403 --> 00:13:28.926 and affects the physical shape of the eye. 00:13:29.736 --> 00:13:32.077 When we put all these pieces together, 00:13:32.101 --> 00:13:35.403 this ultimate explanation for nearsightedness -- 00:13:35.427 --> 00:13:40.458 that environmental and behavioral change impact the way we use our eyes -- 00:13:40.482 --> 00:13:43.426 helps us better understand the proximate cause. NOTE Paragraph 00:13:43.736 --> 00:13:46.474 And an inescapable conclusion emerges -- 00:13:47.029 --> 00:13:48.363 my mother was right, 00:13:48.387 --> 00:13:52.243 I probably should have spent a little less time with my nose in a book. 00:13:53.156 --> 00:13:55.705 This is just one of many possible examples. 00:13:56.047 --> 00:14:00.190 So the next time you or a loved one are faced with a health challenge, 00:14:00.214 --> 00:14:02.317 whether it's obesity or diabetes, 00:14:02.341 --> 00:14:04.023 an autoimmune disorder, 00:14:04.047 --> 00:14:05.888 or a knee or back injury, 00:14:05.912 --> 00:14:07.157 I encourage you to think 00:14:07.181 --> 00:14:09.974 about what an ultimate perspective can contribute. 00:14:10.553 --> 00:14:12.847 Understanding that your health 00:14:12.871 --> 00:14:16.919 is affected not just by what's going on in your body right now, 00:14:16.943 --> 00:14:21.237 but also by your genetic inheritance, culture and history, 00:14:21.261 --> 00:14:23.895 can help you make more informed decisions 00:14:23.919 --> 00:14:27.255 about predispositions, risks and treatments. NOTE Paragraph 00:14:28.489 --> 00:14:29.663 As for me, 00:14:29.687 --> 00:14:32.323 I won't claim that an evolutionary medicine perspective 00:14:32.347 --> 00:14:35.116 has always directly influenced my decisions, 00:14:35.140 --> 00:14:37.007 such as my choice of spouse. 00:14:37.403 --> 00:14:39.101 It turned out, though, 00:14:39.125 --> 00:14:41.545 that not following the traditional practice 00:14:41.569 --> 00:14:43.958 of marrying within the Jewish community 00:14:43.982 --> 00:14:46.572 ultimately worked in my favor genetically, 00:14:46.596 --> 00:14:49.993 reducing the odds of me having a baby with Tay-Sachs. 00:14:50.469 --> 00:14:54.199 It's a great example of why not every set of Ashkenazi parents 00:14:54.223 --> 00:14:57.362 should hope that their daughter marries "a nice Jewish boy." NOTE Paragraph 00:14:57.386 --> 00:14:58.386 (Laughter) 00:14:58.410 --> 00:14:59.565 (Audience) Woo-hoo! NOTE Paragraph 00:14:59.589 --> 00:15:01.339 More importantly, though, 00:15:01.363 --> 00:15:04.301 the experience of learning about my own genes 00:15:04.325 --> 00:15:07.920 taught me to think differently about health in the long run, 00:15:07.944 --> 00:15:11.825 and I hope sharing my story inspires you to do the same. NOTE Paragraph 00:15:11.849 --> 00:15:13.000 Thank you. NOTE Paragraph 00:15:13.024 --> 00:15:14.500 (Applause)