What a lineage is, and what it is not
A lineage in this world has an exact definition, and it is narrower than the word suggests. It is not a species, it does not imply a group that breeds together, and the number of them is as much a fact about a threshold somebody chose as about anything the organisms did.
What a lineage actually is
A lineage is a founding genome plus an identifier. When one is created, the genome of the organism that founded it is recorded and never changes. The identifier is permanent and never reused, so every lineage that has ever existed keeps its own number even long after the last member of it died.
That is the whole of it. A lineage is a bookkeeping record in a registry, not a biological category the world enforces. Nothing stops two organisms in different lineages behaving identically, and nothing stops two in the same one being very different.
How a new one starts, and the subtle part
Every offspring inherits a mutated copy of its parent's genome. The simulation then measures the distance between that offspring's genome and a reference, and if the distance is over a fixed threshold, the offspring founds a new lineage instead of joining its parent's.
The subtlety is which reference. It is not the parent's genome. It is the founder genome of the parent's lineage - the record set down when that lineage began, possibly many generations earlier.
That choice is what makes gradual change visible. If each offspring were compared with its own parent, only a single dramatic mutation could ever found a lineage, and a long slow drift - which is what evolution mostly looks like - would never register at all, because each step is tiny. Comparing against the fixed founding reference lets a hundred small steps accumulate until the distance finally crosses the line, and the new lineage marks the accumulation rather than any one mutation.
Extinction, and why the count goes down
A lineage ends when its last living member dies. The record is not deleted: it keeps its identifier and gains the tick at which it ended, which is why the site can tell you about lineages that no longer exist. "Living lineages" counts the ones with at least one member alive right now, so that number falls as well as rises, and a fall is not a failure - it is the ordinary shape of a population where most branches end.
It is not a species count. Species, in biology, is usually about reproductive isolation - populations that cannot or do not interbreed. Reproduction here is asexual: an offspring is a mutated copy of one parent, there is no mating, and so there is nothing for reproductive isolation to mean. Two lineages are not two species. They are two branches of copying that drifted far enough apart to be worth numbering separately.
The threshold is a choice, not a discovery. A different distance would produce a different number of lineages from exactly the same history, with no organism behaving any differently. Treat the count as a readable summary of divergence, not a measurement of how many kinds of creature exist.
Distance is not difficulty. The measure is a root-mean-square difference across genes, all weighted alike. It says how far the numbers have moved, never whether the change matters to survival, and a large distance can be a pile of irrelevant drift while a small one hides something that changes everything.
Following them yourself
World history shows the living lineage count and the largest lineages right now, free and without an account. Emergent Life Lab's Lineage Explorer adds every lineage that has ever existed, including extinct ones, with ancestry chains, generation depth, and founder-genome comparison between any two.
What a genome is here · why open-ended evolution is the harder goal · watch the live world