How can bone marrow be one of the first human foods? Humans had to learn the use of tools first to access bone marrow. Humans needed a better brain to figure out how to create and use tools.
Also, predators are very intelligent. They can and will break open bones. Domestic dogs do that. Hyenas are renowned for their bone-crushing abilities.
Bone marrow is not the only source of fat. There are plenty of plant-based foods rich in fat
Most - if not all - primates can use simple tools like rocks to crush things. Our ancestors used the simplest of tools even before they were considered human.
Also, predators are very intelligent. They can and will break open bones. Domestic dogs do that. Hyenas are renowned for their bone-crushing abilities.
Bone marrow is not the only source of fat. There are plenty of plant-based foods rich in fat