Sometimes I get curious about something our baby is doing, and 20 minutes later, I'm reading an academic paper entitled
"What Does Prone Skateboarding in the Newborn Tell Us About the Ontogeny of Human Locomotion?"
Link : https://sci-hub.se/10.1111/cdev.13251
I noticed our baby making really poor attempts to move forward by pushing back her feet and waving her arms repeatedly. This behavior started a few hours after birth. I wondered if it was some sort of primordial mammalian crawling reflex that helps newborn quadrupeds make their way over to mom's nipples. Humans have a different infancy from nearly all non-primates because mothers can literally pick up newborns to nurse them, whereas most other newborn mammals (eg puppies, kittens) have to crawl their way over to the teats.
These researchers decided to study reflexive human infant crawling locomotion by building tiny skateboards for premature babies just a couple of days out of the womb, and indeed, they have locomotion patterns very similar to quadruped mammals. They just lack the strength to make it work.
It's another one of these evo-devo situations where more modern patterns (eg walking) are built on top of, and arise later in development, than older mammalian crawling behavior. Most mammals have to start walking within a few hours of birth, and humans have the rare luxury of having months to figure it out.