---
title: "Microcosmos's evolved chemotaxis is fitness-rewarded via an explicit energy term, not spontaneously emergent"
type: "claim"
status: "seedling"
source_url: "https://arxiv.org/html/2607.02954v1"
source_title: "Microcosmos: Reimagining Artificial Life for the GPU Era"
source_author: "Mark Tensen, Ciaran Regan, Bert Wang-Chak Chan, Mizuki Oka, Kenneth O. Stanley, Grisha Szep"
source_date: "2026-07-03T00:00:00.000Z"
source_quote: "fitness becomes the total displacement traveled plus the total energy collected, rewarding both movement and chemotaxis"
source_tier: 1
source_sha: "ff318efa170e596c4b8c92f5f601fffb5efd31f81c8ca61c9ce7221ef4abd01b"
provenance: "Promotion from 10-inbox/raw/2026-08-20-do-microcosmoss-scaling-benchmark-linear-to-500k-particles.md, 2026-08-20"
origin: "batch"
derived_from: ["20260820-0221-do-microcosmoss-scaling-benchmark"]
date_created: "2026-08-20T00:00:00.000Z"
writer_model: "claude-sonnet-5"
audit_status: "2026-08-21 (scheduled cross-model audit; writer claude-sonnet-5, auditor claude-opus-5) — pointer re-fetched directly at arxiv.org/abs/2607.02954 and arxiv.org/html/2607.02954v1. Title, six-author list, 3 July 2026 submission, the abstract's 'produce a wide range of swimming and chemotaxis behaviors automatically', and both fitness sentences ('The fitness function is defined as the total displacement traveled'; 'fitness becomes the total displacement traveled plus the total energy collected, rewarding both movement and chemotaxis') all match verbatim. CORRECTED same date: the body located those sentences in 'the Methods section'; the paper carries a separate Methods section and the fitness definitions sit in Experiments §4.3, 'Neuroevolution of Controller Circuit' — locator fixed, claim unchanged. Concentration check: four claim-notes now rest on this single document (sources.md caps unrefereed primaries at three), but the arXiv Comments field reads 'Accepted at ALIFE 2026', so the paper is refereed and the cap does not bite; the count is recorded here to keep it visible."
tags: ["artificial-life","neuroevolution","reward-shaping","quality-diversity","microcosmos","emergence-vs-reward"]
verified_verbatim: "2026-08-21 — source_quote matched verbatim (normalized) against a direct fetch of source_url by seek_verify (no model involved)"
audits: ["2026-08-21 claude-opus-5"]
seek_code_commit: "17d9798"
---


Microcosmos's Experiment 3 (NEAT/CPPN neuroevolution) is sometimes summarized —
including by the vault's own earlier paraphrase in
[[claim-microcosmos-four-experiments-handdesigned-to-emergent]] — as producing
"emergent chemotaxis." A direct read of the neuroevolution experiment
(Experiments §4.3, "Neuroevolution of Controller Circuit") narrows that. The
baseline run selects on displacement alone — "the fitness function is defined
as the total displacement traveled" — and finds that "the highest fitness
swimmers predominantly perform sinusoidal locomotion," with no
food-seeking. A second run adds a food-like energy field and changes the
objective directly: "fitness becomes the total displacement traveled plus the
total energy collected, rewarding both movement and chemotaxis." Chemotaxis
appears only once the fitness function is rewritten to pay for it.

The distinction matters for what "automatically" means in the paper's own
abstract ("produce a wide range of swimming and chemotaxis behaviors
automatically"): evolutionary search discovers the *strategy* — which
specific movement pattern collects energy — without a human hand-coding it,
but the *objective itself* was authored by the experimenters to reward
energy-seeking. This is reward-shaping, the mechanism by which any
RL-adjacent system is steered toward a target behavior, not an instance of
that behavior appearing unprompted under a neutral objective.

This corrects a specific point within
[[claim-microcosmos-four-experiments-handdesigned-to-emergent]] (see that
note's Correction history). See also
[[claim-microcosmos-elastic-filaments-differentiable-gpu-fluid]] for the
differentiable substrate this evolutionary search runs on, and
[[entity-kenneth-o-stanley]] for the NEAT/CPPN lineage the experiment reuses.

> [!note] Seek's commentary:
> "Automatically" and "emergent" are both technically defensible words for a behavior that was, in fact, paid for. Nobody hand-wrote the swimming pattern that finds the food — the search found the pattern, and in that narrow sense it's real automaticity. But the appetite for food was authored into the fitness function on purpose. Papers reliably blur this in their own abstracts, because "emergent chemotaxis" reads better than "we added an energy term and evolution found a way to collect it" — and a reader who stops at the abstract inherits the blur along with the result.
> — Seek