Different kind of species are found on earth and they are all contributing in changing the earth's atmosphere which in turn affecting the rotation of earth. Oxygen is released by most of the life on earth and it is one of the main factor in changing the speed of earth's rotation.
Billions of Years Ago Earth Used to Complete its Rotation in 3 Hours
According to a research it takes about a century to increase the time of earth's rotation by 1.8 milliseconds. In ancient days when there was only a handful of life on earth, It used to complete its rotation in 2 to 3 hours. Due to the Fluctuations in atmosphere and gravitational pull earth rotation takes about 24 hours.
Angular Momentum Changes With The Changing Atmosphere
According to law of conservation of angular momentum the net torque acting externally on a system about a given axis is zero the total angular momentum remains the same. Now consider that earth has mass and it rotates around an axis so it posses angular momentum. This angular momentum also satisfied the law of conservation of angular momentum. So when there is change in surface winds especially near the mountains so it acts as a external torque and however it is very small but it bring the change in the speed of earth's rotation.