Did Childhood Come Before the Big Human Brain?

What the new review is claiming

The September 2026 Developmental Review paper does not report a new excavation or a new fossil. It is a synthesis. Its central argument is that the usual causal story—larger brains forced human childhood to become longer—is incomplete. Tobias Grossmann argues that prolonged development can be seen in hominins that did not yet have modern human brain size, so the first extension of childhood may have preceded major encephalization rather than simply followed it.

Why the classic brain-first story looked reasonable

Large brains are metabolically expensive, take longer to grow and mature, and modern humans combine unusually slow development with very large brains. That made it plausible to treat extended childhood as a downstream consequence of encephalization. The new review does not reject that later feedback. It argues instead that the fossil record makes the starting sequence less tidy: slower development may already have been present before the largest increases in brain size.

Australopithecus already complicates the sequence

A 2020 Science Advances study of Australopithecus afarensis endocasts reported ape-like brain organization alongside evidence for prolonged brain growth. The species retained a brain size much closer to other apes than to later Homo, yet its developmental timing appears to have been extended. That is difficult to fit into a model in which only a large brain creates the need for a long childhood.

A 2026 fossil study pushes the question deeper into Homo

In July 2026, Yousuke Kaifu and colleagues proposed deformational plagiocephaly as a skeletal marker of helpless infancy. In a comparative dataset of 1,504 crania, they argued that marked forms of this deformation are strongly associated with the immature cranial and muscular condition of human infants. They then reported comparable deformation in some Indonesian Homo erectus and Homo floresiensis fossils, which they interpret as evidence that highly dependent infancy may have deep roots in the human lineage.

That fossil marker is evidence, not a stopwatch

The plagiocephaly study is important because it tries to infer infant dependency from fossil anatomy without needing a preserved infant of known age. But the inference is still indirect. A cranial deformation is not itself a measured childhood duration, and fossil distortion, individual variation and the representativeness of a small number of ancient specimens remain reasons for caution. The study supports a deep origin for helpless infancy; it does not date a full modern-human life-history schedule.

Neanderthals provide a useful counterexample

The 2026 analysis of Amud 7, the most complete Neanderthal infant skeleton in the roughly 6–14 month age range, found a mismatch between developmental clocks. Dental evidence suggested an age around six months while skeletal and endocranial growth was more comparable to substantially older modern human infants. The authors interpret this as unusually rapid early somatic growth, with similar patterns noted in other rare Neanderthal juveniles.

Why the Neanderthal result matters to the argument

Neanderthals had brains comparable in volume to or larger than modern humans, yet the Amud evidence points toward faster early growth. If that pattern is representative, brain size alone cannot dictate developmental pace in a simple one-way relationship. That strengthens the review’s broader point: large brains, childhood length, energy budgets and social care likely coevolved rather than lining up as one simple chain.

The social hypothesis: childcare as enabling infrastructure

Grossmann’s Childhood-First Hypothesis places cooperative care upstream. The idea is that extensive allomaternal or shared childcare could reduce the survival and energetic costs of dependent young, allowing slower development to persist. Once that social niche existed, prolonged learning and later brain expansion could become easier to sustain. Related work on cooperative breeding reaches a similar conclusion, treating extensive childcare as a plausible catalyst rather than merely a consequence of big brains.

What would make the hypothesis stronger

The strongest tests would connect multiple independent clocks in the same fossils: dental development, endocranial growth, skeletal maturation, pelvic and neonatal constraints, and securely dated life-history indicators across several species. More infant and juvenile fossils are especially important. A convincing causal case also needs chronology: evidence that prolonged dependency consistently appears before major encephalization in multiple lineages, not just isolated examples.

The evidence-led conclusion

The new review has a real point: the fossil record no longer supports a clean assumption that big brains had to come first and long childhood followed automatically. There is credible evidence for prolonged or highly dependent early development in hominins with smaller brains, while Neanderthals may have combined large brains with faster early growth. The strongest current reading is therefore not ‘childhood definitely caused big brains,’ but that human life history and brain expansion evolved through a more entangled social and biological process than the classic one-way model suggests.