From an AP (attachment parenting) point of view...there's speculation that slings or baby carriers were "invented" very early, at about the same time as clothes, so babywearing and sling-carrying ARE part of human evolution. Most animals can be divided into "carriers" and "hiders". Some hide their babies from predators and those animal's young will freeze and be silent when separated or frightened. Some animals carry or otherwise keep their young with them always, and those babies cry or bellow when separated from mother. It's obvious that human babies are biologically primed to cry when not in physical contact with a parent (just like most other apes) so it makes sense that a baby being rocked in a way that MIMICKS a mothers arms will settle. Yeah, just buy a sling.

I really like the slings so that baby is right next to you.

Actually Eric's answer is relevant. A sling is one of the ways to keep a baby attached to the body so that the movements of the body will rock the baby to sleep or help a baby calm by providing warmth and comfort. There are many ways to "wear" a baby, all of which provide soothing motion. Babies, some more than others, seem to need to be weened from the womb slowly and rocking motions, especially coupled with closeness and warmth, are natural states that the baby is used to. Even our primitive ancestors probably figured out ways of strapping babies to themselves so that their hands were free and they could get work done.

So yes, to answer your question, being still is not a natural state for a baby. Motion helps. But motion + warmth + heartbeat = calm baby and adult who can get things done.

Well, if you approach it evolutionary (and I'm no expert, just presenting my thoughts:

Why would a baby cry? What would happen with babies that don't cry?

If a baby cries it obviously wants something. If a baby would cry all the time, predators would notice that rather quickly, so they would die. A baby cries for food and such. So why would it cry when it doesn't get attention or is, as you suggest, 'on the move'?

Perhaps it is not so much being 'on the move', but more knowing that the mother is close. The crying might be a reminder for the mother to not forget or lose the child. The child 'knows' if it is moving somehow or being touched, it must be close to a human/the mother...

So you might let your kid know, by shaking it around somehow, "I'm still here, you'll survive. It's okay!"

Maybe...

To answer "Cave Man Mind"'s "why? and add to Rhi's awesome answer, some modern parents believe in the Nine-In-Nine-Out idea, where baby is in the tummy for 9 months and then in a sling/wrap/etc for another 9. Another term for this is the "Fourth Trimester."

Personally, I use slings because I'm too lazy to drag a stroller in and out of the car and I think people dragging bucket carseats around, bouncing off their knees because they are so heavy look ridiculous!

Man! I love Rhi's answer!

Some mammals, like bunnies for example, only nurse their young twice/day. Human newborns need to nurse at least every 2 hours in the beginning at least, so they really can't be plopped in a nest and left while CaveMommy goes hunting for berries. If CaveMommy goes too long without nursing her breasts will get sore and maybe even lead to infection so it's easiest to just keep baby with her to facilitate easy breastfeeding.

Experts say human babies are born with just three basic reflexes: sucking, swallowing and breathing -- and even breathing can be irregular. This may have to do with the immaturity of the human newborn's brain, which is only about 25 percent of its adult weight at birth, while most other mammals are born having 60 percent to 90 percent of their adult brain size.

Some of the answers are found in wikipedia's babywearing answers: Benefits of babywearing include:

Mothers' oxytocin is increased through physical contact with the infant, leading to a more intimate maternal bond, easier breastfeeding and better care, thus lowering the incidence of postpartum depression and psychosomatic illness in the mother.[2]
Infants who are carried are calmer because all of their primal/survival needs are met. The caregiver can be seen, heard, smelled, touched, tasted, provide feeding and the motion necessary for continuing neural development, gastrointestinal and respiratory health and to establish balance (inner ear development) and muscle tone is constant.[3]
Infants are more organized. Parental rhythms (walking, heartbeat, etc.) have balancing and soothing effects on infants.
Infants are "humanized" earlier by developing socially. Babies are closer to people and can study facial expressions, learn languages faster and be familiar with body language.[4]
Independence is established earlier.[3]
Attachment between child and caregiver is more secure.[5]

To add to craftycrofts ...

Homo sapien babies are born too early. It's an evolutionary quirk caused by the fact that a homo sapien female cannot gestate her offspring to the same level of offspring development that most other mammals do because, to be blunt, she would simply not be able to expel the baby (the head would be too large).

So she gives birth early, before the baby has the ability to walk etc. This is why human babies are so hopeless; theoretically, they should still be in utero.

Add to this the hypothesis that pregnant Neanderthal females underwent a twelve month gestation period with their young, and that apparently quite a lot of Europeans carry Neanderthal DNA, and the picture starts to look very interesting indeed.

Essentially, you could argue, if you accept the above, that humans are somewhat like kangeroos. The human baby is born and is evolutionarily designed to be carried by the mother until it develops to more independent stage of development. This is possibly why human babies desire motion, warmth, and "in utero" sounds and experiences (swaddling etc).