Missing Link Series by Kate Thompson

The first book in a trilogy about terrifying genetic experiments. Danny is different. He's slower than other kids his age, and prone to violent outbursts. But when Danny runs off to find his biological mother, his older stepbrother Christie impulsively follows him. Along the way the two boys are joined by a runaway girl, a bird, and a dog that can speak to humans. However, nothing can prepare them for what they learn when they finally locate Danny's mother at her laboratory, Fourth World... moreThe first book in a trilogy about terrifying genetic experiments. Danny is different. He's slower than other kids his age, and prone to violent outbursts. But when Danny runs off to find his biological mother, his older stepbrother Christie impulsively follows him. Along the way the two boys are joined by a runaway girl, a bird, and a dog that can speak to humans. However, nothing can prepare them for what they learn when they finally locate Danny's mother at her laboratory, Fourth World. It seems Danny's mother has been performing experiments - secret experiments - with terrifying genetic significance... less

Christie and his friends continue their search for the missing link in the faraway Himalayas, where the answers may be found with the elusive and mysterious Yeti. Their journey leaves Danny battling his overwhelming longing for the sea, and Danny's half-sister Sandy fighting for her father's approval. The experience will bring out the best and worst in the group and eventually compel each of them to ask the ultimate question: What does it mean to be human?