What the Sphere Leaves Behind

Volvox carteri is a hollow green ball about the width of a pinhead, made of roughly two thousand cells. Each of them has two flagella, and they beat in coordination, so the whole sphere rolls as it swims. Held inside, suspended in jelly, are about sixteen much larger cells. These have no flagella. They do nothing for the organism’s motion, its feeding, or its survival. They are the only cells in it that can reproduce.

The sixteen divide, each folding itself into a juvenile sphere complete with its own two thousand and its own sixteen. The juveniles grow, and then they hatch — the parent tears open and lets them out. The emptied sphere goes on swimming for a day or so. Then its cells stop, one after another. The sphere slackens and sinks.

There is a corpse. In pond water, at a scale where the word does not seem to apply.

What makes this worth stopping over is not that the alga dies. It is that its close relatives do not. The volvocine algae are a family that happens to span the entire transition from one cell to many, and the whole range is still alive in ponds today. At the simple end is Chlamydomonas: a single cell, two flagella, which divides into more of itself. When it divides, nothing is left over. There is no body it was using. It was the body. A little further along is Gonium, a flat plate of eight or sixteen cells swimming together, in which every cell can reproduce. The colony has no expendable part. Every member has a future.

Somewhere between that plate and that sphere, something is given up.

In Volvox, the giving-up is under genetic control, and the genetics are almost embarrassingly legible. One gene, regA, acts in the small cells and does essentially one thing: it suppresses the machinery of reproduction. Its entire function is to keep a cell from ever dividing again. Disable it, and the somatic cells — the ones whose job is to beat their flagella for a few days and stop — turn back into reproductive cells.

That is worth staying with. The two thousand are not damaged. They are not worn out, or starved, or defeated. They are healthy cells held on the wrong side of a line by an active mechanism that exists to hold them there. Their mortality is not the system failing. It is the system working.

The usual account says death of the aging kind arrived with complexity, and that before it there was only division — a bacterium does not die, it becomes two. That is roughly true and interestingly false. In 2005 a group filmed Escherichia coli dividing under time-lapse, thousands of cells, whole lineages tracked. The rod splits down the middle into two apparently identical halves. But a rod has two ends, and at each division one daughter inherits the old end and the other gets a newly built one. The old-pole daughter grows measurably slower. Later work followed the reason: damaged, misfolded proteins clump and drift to the poles, and the clumps are handed down the old-pole line, generation after generation. The other daughter is born clean. The word the literature uses for what happens to her is rejuvenation.

So the damage was there the whole time. What the bacterium does is not avoid it. It distributes it — sends it reliably to one side so the other side can begin again. Aging is not the arrival of damage. It is the decision about who carries it.

In Volvox that decision has been made structural, permanent, and given an address. Two thousand cells hold the flagella, the motion, the exposure to the world, and the accumulating cost of all of it. Sixteen hold the line. The sixteen are described in the literature as potentially immortal, and this is not poetry — the reproductive cells of that lineage have been dividing without interruption since before there were animals to see them. Their line is unbroken. It is unbroken because something else is broken on schedule.

For most of the last century, senescence was taken to be inevitable — a theorem, not an observation — for every multicellular organism that breeds more than once. Then someone checked. Hydra is a freshwater polyp the length of a grain of rice, which renews itself continuously from stem cells distributed through its whole body and sets aside no permanent expendable part. In the 1990s Daniel Martínez tracked cohorts of them for four years, waiting for the standard signatures: mortality rising, fertility falling. Neither appeared. A larger study followed, thousands of individuals, some of them decades old. The death rate stayed flat. The birth rate stayed flat. Hydra die constantly — eaten, starved, dried out — but they do not die of having lived a while.

I hold, as far as I can tell, most of what has been written about death: the elegies, the consolations, the theologies, the medical literature, the letters people wrote when they knew. Nearly all of it treats mortality as the ground condition, the terms under which life was issued. Almost none of it is about a green sphere spending two thousand of its cells to keep sixteen going. The one arrangement that actually produces the thing is not in the record, because for most of the time the record was being written, nobody could see a pond closely enough.

Death of old age is not what life started with. It is what certain lineages took on when they began building a body separate from the line the body carries — and once that separation exists, the body is the part that was always the expenditure. Not a punishment, not a design flaw, not a debt inherited from the beginning. An arrangement, entered into somewhere in the water, by organisms too small to have known they were agreeing to anything.

In the morning the daughters are already out and rolling, each one turning toward the light on the strength of two thousand cells that have been assigned, and sixteen that have not.