It certainly wasn't early physical ability. My twice-exceptional son was nothing like my former cheerleader daughter who walked and crawled early or like his cousins who get a lot of attention for their sports ability in our small town or even his dad who played football in the army and at age 59 is still very physically fit. My son couldn't draw or color in the lines or manipulate puzzle pieces very well and he refused to do these things out of frustration. As a baby he seemed floppy and he lacked the strength and stamina to do things the average baby could do physically. At 12 months he could not crawl or pull himself up or even stand for very long while holding on to something. The physical therapist who tested him at 12 months told us he was at the 6 month old level for physical skills.
What he could do well, even from the time he was a few months old, was communicate what he wanted. At 12 months his receptive and expressive language skills were 6 months ahead according to the speech language pathologist who tested him. So six months ahead in one thing and six months delayed in another. I remember thinking if we averaged those scores he was a normal 12 month old.
At 12 months and even months before that he was interested in everything around him and demanded to know "is dat" all day long. He couldn't move well by himself but he wanted me to carry him through the house so he could ask me what is dat about everything--even the spice rack with individual spices. He would point to one and I had to read it. He laughed every time I read "Tarragon" and I think he found it funny because his sister's name is Tara and he heard "Tara gone." He was recognizing individual letters and had favorites and would get really excited when he saw the letters A, B, P or Z on signs or anywhere. He loved books, especially alphabet books and he counted with the Teletubbies. My daughter, who read at 4, was never that interested in books. She liked books but she did not love them. I had never before seen a baby get so excited over books. Because it made him so happy and excited to learn things it was fun to show him new things.
So I guess I would have to say the insatiable curiosity and communication skills made him seem different.