He posed the question this way:
What are the chances that, if ever we do make contact with intelligent beings which have evolved under similar conditions, they too will be upright, bipedal animals with two arms, each ending in a five-fingered hand, and a head mounted on top of the body carrying a pair of eyes, a nose and a mouth?By intelligent beings, he is referring to the kind that "builds civilizations and spaceships." By studying the evolutionary history of mankind, he came to the following conclusions.
1. It must have a large brain in proportion to its body: This would allow it the ability to think and to adapt to its environment as necessary.
2. It must not be too big: An elephant is so huge "it is insulated from the dangers of attack by enemies and has never had to use its intelligence to fight off enemies.
3. It must not be too fierce: Animals like tigers, lions, and bears have sharp fangs and claws, and have the ability to rip enemies to shreds and eat them. They have no reason to be intimidated by enemies, nor any need to run and hide.
4. It must not be too good at hiding: Rats and squirrels are small and fast enough that they could easily scurry away from danger.
5. It must not be too comfortable in its environment: The whale and the dolphin are considered to be as intelligent as humans by many, but they "have few enemies and are supurbly adapted to their watery environment."
6. It must live on land: A dolphin-like or whale-like creature would be too comfortable with its environment to make any changes. Therefore, "Intelligent, tool-using life will emerge on a planet like ours on the land, not in the sea."
7. It must have two legs for running: This would have to be able to use these appendages to run from enemies, and to hide, and to wait for a chance to adjust to their new environment.
8. It must have two arms with hands: This would allow it to carry things. The number of appendages does not matter, as many people do just fine with only two fingers. However...
9. It must have a thumb: This allows it to grip things, and put together the tools needed to adapt to its new environment.
10. It must have a head mounted on top with two eyes: This would give it a "three-dimensional view of the world. Three-dimensional vision is esential for judging distances, whether it be the distance to a charging lion or to a morsel of food waiting to be picked up."
11. Eyes mounted high above the ground: This "is essential for a prey animal, which needs early warning of impending danger.
12. It must have something resembling a mouth and nose: This will allow it to breathe air and to eat food.
All of these may come in various forms. For instance, while the intelligent life form must have two legs, it may also have a tail for balance, like a kangaroo or a Saurornithoides.
Imagine, for instance, that saurornithoides had followed the same evolutionary path, in response to similar evolutionary pressures, as humans later did. A human traveling to such a planet would be an impressive civilization of "eight-fingered, kangaroo-like bipeds."
Of interest here, that humans developed an arithmetic system using a base of ten. We must assume this is because humans have five digits on each hand, and ten total digits. Since saurornithoides have four digits on each hand, and ten total digits, it would have developed an arithmetic system with a base of eight.
Of course the main sticking point to any of this is whether God would allow for it to happen. There are many Christians who refuse to believe in evolution, although, to me, there is too much evidence to deny it. Therefore, I see no reason why evolution and Christianity cannot live side by side.
I also see no reason why a loving God would not allow an intelligent life-form to evolve on a distant planet. Or, at the very least, the theories postulated by Gribbin should help some writer of fiction establish a distant civilization on a planet named Alton.