Human babies are born not able to move by themselves. The only movement they know is their sucking reflexes. They develop how to move muscles over time. It's not as easy for babies to easily just grasp a spoon if they wanted to. Learning about the body and making it do things you want it to do takes time. It is somehow programmed in our mind to learn to accomplish simple motor development first and then learn the more complex one afterwards.
The major milestones in motor skill development are sitting up, crawling, standing unsupported and walking. Sitting without support usually occurs around 6 months, crawling usually occurs around 9 months, standing occurs around 11 months, cruising occurs around 12 months and walking without assistance occurs around 13 months. The age at which different children reach these motor milestone varies. Studies has shown that physical maturation plays a major role in allowing children to achieve the motor milestones. Bigger babies tend to develop motor skills earlier due to more muscle mass build up compare to smaller, lighter babies. Although the age at which they acquire these skills may differ among babies, almost all acquire them in the same order, from simple to complex.
My Nolen is only 4 months old and he has already began to sit by himself. I'm so proud of him. My husband and I were so impressed by how fast he learned how to do it. He has a cousin who is around the same age as he is and she's not able to sit yet. I understand that Nolen may have learned to sit first but it doesn't correlate to him being smarter than his cousin because every baby is different and also Nolen's weight plays a major factor into why he's already sitting. Nolen also has a cousin who did not learn to sit until she was 11 months. She learned how to roll over first because being able to sit. Every time we see her she's always rolling on the ground. She estimates the angle at which she needs to roll to get where she wants to be. I find it so cute and funny how she used to do that. She serves as an example that not all babies learn these motor skills in order. Down below I have a link to an article on babycenter.com that goes in depth on motor development skills. When I was pregnant I signed up for babycenter.com and I love how they sent emails on how your baby is developing. They have great resources. I highly recommend parents and parents-to-be to sign up! www.babycenter.com