Lesson 30 — When cells stop competing — the body emerges UNIT 5 CORE · cell → individual
Your body is a truce between cells that could each still be reproducing on their own.
Your body is a bee colony. The gametes reproduce. The somatic cells — muscle, skin, gut, neurons — help the gametes survive. All your cells are 100% related to each other. Each gamete is fully related to each other cell in your body. What your body is, is basically a bee colony. We have self-divided roles. Multicellularity is just kin selection on cells. Same math.
— 461_lec29_02
A — Cells living on their own
You are not the pond water you looked at in the microscopy lab. In pond water, what were those little cells doing to each other? Murdering. They were murderers trying to kill each other. Your cells don't do that, normally. As a result, they can cooperate with one another. They can have something to push against. They can build extracellular matrix. The fibronectins and integrins are how a cell anchors and pushes — because in you, things stay still long enough for that to work.
— 145_lec09_05
The same 200 cells, but nothing groups them. Each one reproduces for itself. Move the slider and watch which term has anything in it.
within (each cell for itself):— between (colonies):— the unit:—
With no colonies there is nothing for a between-term to be computed over, so it is not small — there is no second level for it to live at. This is the baseline the next stage breaks.
B — Add the colony: germ and soma
Your neurons cooperate because they're nearly genetically identical. The cost to a neuron to help another nearly identical, almost perfectly related neuron isn't actually a cost — genetically speaking. They're basically the same. That's why thought is possible — your neurons aren't trying to destroy each other.
— 202_lec30_03
200 cells in 10 colonies of 20. Each cell has a selfish-growth trait z and a fitness w; each colony has a fitness of its own. Grey dots are cells, red dots are colonies.
Predict first. Cells in a colony start out unrelated and you make them steadily more alike. The level carrying the change will end up being:
within-colony cov(w,z):—
|
between-colony cov(W,Z):—
|
total cov:—
which level is the unit? —
Red points: each colony's (Z, W) — the between-level data. Gray: individual cells' (z, w) within colonies. When the red point cloud is more correlated than the gray, the colony is the unit and the cells are parts. When the gray correlation dominates, the cells are still in business for themselves.
C — The diagnostic: when is the colony the unit?
A big part of being a multicellular organism — kind of the definition — is that your cells are not growing as much as they could. For the betterment of all the other cells. All your cells are choosing to make fewer copies of themselves individually than they could, in order for the organism as a whole to work. This is a really difficult and problematic thing for a cell to do, because if you make more of yourself, you become more common — no matter why you're making more of yourself. Which is how you get cancer.
— 145_lec22_04
The diagnostic is just the ratio cov_between / (cov_within + cov_between) — the same between/total quantity you computed as F_ST in Lesson 14, now applied to a fitness covariance instead of an allele frequency. Same machine, new level. Diagnose four real systems:
Honeybee workers. Within-colony cov(worker reproduction, fitness) ≈ 0 (workers don't reproduce). Between-colony cov(colony productivity, colony survival) is large. Ratio ≈ 1: the colony is the unit.
Your soma. Within-body cov(cell-level z, cell-level w) ≈ 0 (cells don't compete with each other for reproductive turns). Between-body cov(somatic phenotype, fertility) is the entire story. Ratio ≈ 1: the body is the unit.
Volvox. Some cells are sterile, some reproduce. Between-colony cov dominates by a factor of ~10. Ratio ≈ 0.9: the colony is the unit, with measurable but small within-colony slack.
Cancer. A somatic-cell lineage that has re-evolved its own cov(z, w). Within-body cov re-emerges. Ratio drops sharply: the body's "individuality" is failing.
D — Three empirical anchors
You can only inherit cooperation, because cooperation is the thing that persists. The lineages whose cells fought each other into oblivion didn't make it. But your body is de novo inventing ways to try to break you down. That's cancer, that's disease, that's ailment. We constantly inherit the framework of cells working together. But the cells inside us are constantly inventing new ways to defect.
— 202_lec30_05