Why think about family composition when we’re talking about longevity? Because, in addition to being an inventive species, we are a caregiving species. Our relationships matter. The care we do—or do not—give one another is a big factor in how long we live. Married people tend to be healthier and to live longer. This will remain true in a world where organs can be grown in Petri dishes or spinal cords regenerated from stem cells. Somebody still has to take you to your third artificial heart replacement and drive you home from the hospital and comfort you if it hurts and make sure you get the right mix of pain meds and anti-inflammatories and that you do your physical therapy. Even if babies are someday conceived and gestated in a laboratory, somebody must care for infants to ensure they grow up to be loving, empathetic, mature human adults. There is an intriguing theory, the grandmother hypothesis, that attempts to explain why human females live past their reproductive years, a trait we share with very few species, among them killer whales. The hypothesis holds that the evolutionary purpose of grandmothers is to forage for grandchildren, and watch them when the parents are gathering and hunting. Cultures that utilized grandmothers to caregive, this theory goes, were the ones that enjoyed a spike upward in longevity.

And here’s the thing about caregiving: It doesn’t get more efficient with time. Unlike computer technology, human caregiving doesn’t double in processing power every two years. It remains slow and labor-intensive. Children require an enormous amount of care, and so will we, as we expand our lifespans to the next century. We will require physical care, but also emotional and psychological care. How we experience life as we approach 120 or 150 depends on the technology we create, and our access to that technology, but it also depends on our access to people we love and who love us back. What is the future of human pair bonding? Serial monogamy? Serial cohabitation? The death of marriage? A world of people who are 150 and living alone? Will assisted living, the private sector, replace the care of a spouse? Who are the close companions of tomorrow? Will a new kind of marriage evolve? Now that we have largely moved from an idea of marriage as procreative to marriage as companionate, will there be a new model: the caregiving marriage?