人类进化的太突然了 我为什么存在? 祖母才是人类进化的关键

我为什么存在? 祖母才是人类进化的关键  

Why do I exist? This isn't a philosophical cri de coeur; it's an evolutionary conundrum. At 58
I'm well past menopause
and yet I'll soldier
on
with luck
for many years more. The conundrum is more vivid when you realize that human beings (and killer whales) are the only species where females outlive their fertility. Our closest primate relatives--chimpanzees
for example--usually die before their 50s
when they are still fertile.
This isn't just a miracle of modern medicine. Our human life expectancy is much longer than it used to be--but that's because far fewer children die in infancy. Anthropologists have looked at life spans in hunter-gatherer and forager societies
which are like the societies we evolved in. If you make it past childhood
you have a good chance of making it into your 60s or 70s.
It turns out that my existence may actually be the key to human nature. This isn't a megalomaniacal boast but a new biological theory: the 'grandmother hypothesis.' Twenty years ago
the anthropologist Kristen Hawkes at the University of Utah went to study the Hadza
a forager group in Africa
thinking that she would uncover the origins of hunting. But then she noticed the many wiry old women who dug roots and cooked dinners and took care of babies (much like me
though my root-digging skills are restricted to dividing the irises). It turned out that these old women played an important role in providing nutrition for the group
as much as the strapping young hunters. What's more
those old women provided an absolutely crucial resource by taking care of their grandchildren.
原来，我的存在实际上可能是人类本性的关键。这不是狂妄自大的吹牛，而是一套新的生物学理论：“祖母假说”(grandmother hypothesis)。20年前，犹他大学(University of Utah)的人类学家克里斯滕・霍克斯(Kristen Hawkes)去非洲研究狩猎采集族羣“哈扎”(Hadza)，以为她会揭开狩猎的起源。但去了之后她注意到很多身材精瘦的老妇人负责挖植物的根、做饭、照顾孩子（跟我很像，不过我挖植物根的技能仅限于掰鸢尾根）。原来这些老婆婆对于为集体提供营养发挥着重要的作用，不亚于那些年轻强壮的狩猎者。另外，老婆婆们照管孙辈，提供了一种绝对重要的资源。
There are many controversies about what happened in human evolution. But there's no debate that there were o dramatic changes in what biologists call our 'life-history': Besides living much longer than our primate relatives
our babies depend on adults for much longer.
Young chimps gather as much food as they eat by the time they are 7 or so. But even in forager societies
human children pull their weight only when they are teenagers. Why would our babies be helpless for so long? That long immaturity helps make us so smart: It gives us a long protected time to grow large brains and to use those brains to learn about the world we live in. Human beings can learn to adapt to an exceptionally wide variety of environments
and those skills of learning and culture develop in the early years of life.
But that immaturity has a cost. It means that biological mothers can't keep babies going all by themselves: They need help. In forager societies grandmothers provide a substantial amount of child care as well as nutrition. Barry Hewlett at Washington State University and his colleagues found
much to their surprise
that grandmothers even shared breast-feeding with mothers. Some grandmoms just served as big pacifiers
but some
even after menopause
could 'relactate
' actually producing milk. (Though I think I'll stick to the high-tech
21st-century version of helping to feed my 5-month-old granddaughter with electric pumps
freezers and bottles.)
Dr. Hawkes's 'grandmother hypothesis' proposes that grandmotherhood developed in tandem with our long childhood. In fact
she argues that the evolution of grandmothers was exactly what allowed our long childhood
and the learning and culture that go with it
to emerge. In mathematical models
you can see what happens if
at first
just a few women live past menopause and use that time to support their grandchildren (who
of course
share their genes). The 'grandmother trait' can rapidly take hold and spread. And the more grandmothers contribute
the longer the period of immaturity can be.
So on Mother's Day this Sunday
as we toast mothers over innumerable Bloody Marys and Eggs Benedicts across the country
we might add an additional toast for the gray-haired grandmoms behind the scenes.