Throwing the longevity switch
If you could choose a longer, healthier life, but only by having fewer kids, would you? What if you could eventually have the same number of kids, but only by having sex more often, and with no possibility of becoming a parent as a teen-ager?
Is this really possible? Based on the paper we published last week, we are pretty sure it is, although we don't yet know how much of an increase in lifespan is achievable, nor how much it will "cost" in reduced fertility.
A key assumption is that there are tradeoffs between longevity and reproduction, especially early reproduction. There is plenty of evidence for this antagonistic pleiotropy hypothesis: some gene variants that increase longevity nonetheless stay rare, because individuals with those variants have fewer kids. There are many possible reasons for this tradeoff. Calories used for reproduction aren't available for maintaining our bodies. Blood pressure and insulin levels optimal for reproduction are unlikely to be exactly optimal for longevity. Other risks associated with reproduction include sexually transmitted diseases and direct risks of childbirth. When there is a conflict between reproduction and longevity, natural selection will often favor reproduction.
There are, however, two ways we may be able to choose differently, increasing longevity at the expense of (potential, but maybe not actual) reproduction. First, once germ-line gene therapy is perfected and available (initially, perhaps, only in one or two "outlaw states"), maybe we could reverse some of the effects of past natural selection. We might be able to produce genetically engineered kids who would reach puberty later and with low enough intrinsic fertility that occasional unprotected sex would rarely lead to pregnancy, but who would still be healthy at age 100.
Second, what about people already born? Is there some biological "switch" we can throw, that tilts the longevity-vs.-reproduction tradeoff more towards longevity? Or has past natural selection welded the switch in the "reproduce now" position?
We think the switch is free to move, depending on environmental cues that affected our ancestors' survival and reproduction. Our paper shows that the switch position that maximizes Darwinian fitness depends on whether the overall population is increasing or decreasing. If population is decreasing, then individuals that live longer and reproduce later can contribute a larger fraction to their species' (shrunken) gene pool than those that reproduce earlier, on average, even if a few of them die before they get a chance to reproduce, and even if their lifetime reproduction is less than they might have achieved earlier.
Therefore, even though gene variants that always sacrifice early reproduction to increase longevity may not have persisted in the gene pool, variants that delay reproduction (thereby increasing longevity) only when populations were decreasing are likely to be with us, in each of our DNA molecules, today.
If this is true, all we need to do to increase our longevity is to give our bodies (false) cues that, over our evolutionary history, usually predicted population declines. To the extent that population declines were caused by food shortage, eating less may work, as it does in most species tested. Eating "famine foods" (leaves rather than meat, maybe) may also trigger physiological responses that reduce fertility but extend lifespan. On the other hand, if population declines were usually caused by cold winters, is there some reasonably comfortable way to trigger similar responses?
Delaying reproduction can only increase fitness if it increases the chances of surviving the famine or cold winter and reproducing later. So stresses that often predicted the death of the stressed individual (those associated with violent conflict, perhaps) won't necessarily delay reproduction or increase longevity. But there are lots of examples of mild stress increasing longevity. These stresses presumably trigger health-and-longevity-promoting mechanisms, but we may be the first to explain why such beneficial mechanisms aren't turned on all the time: they tend to reduce fertility.
Now, here's a question for you: would increasing human longevity be a good thing? I've seen this issue discussed in various places, but rather superficially. Assume that this option was made available to everyone, given that the cost could be quite low: inexpensive drugs or lifestyle changes that might even save money. Death rates would go down, in the short run, but so would birth rates, especially in countries where birth control is now rare. Death from old age is a fairly small component of overall population trends in these countries (relative to birth rate and infant mortality), so their rate of population increase might actually slow. But, if people expected to live longer, would they have more children (despite lower intrinsic fertility) or fewer, and at what age? Assuming some increase in population, we might need to grow more food -- a significant challenge -- but how would the overall impact of two healthy 90-year-olds who are still working (perhaps as doctors or nurses) and driving compare to that of one 90-year old who doesn't drive but needs expensive medical care? If professors keep working into their 90's, will that slow the spread of good new ideas, or only of stupid ideas that younger faculty may not know were debunked long ago? Would a longer-lived population produce too many bloggers?