It is necessary for you to have detailed knowledge about the biogenesis of the life on earth before you start to do this exercise. You need this lore to create an individual path of evolutionary states from homo sapiens sapiens back to the first primeval protozoa. You should choose one or more representative beings from each period of the short history of life, some of them may turn out to be new spiritual allies.. Have a close eye on the evolutionary relationship of the chosen species. A aviatic atavism should be chosen for a much earlier period of time than a mammal atavism, for example. Because there are many gaps in the knowledge about the evolution of the species you can invent some fantastic combinations or gigantic forms of now small animals (The sabertooth tiger, mammoth and the like are prehistoric examples, widely known). Do you know that there are animals living today, which evolutionary history brought them back to the sea? Or that there was a giant bird alive, back more then 100 years ago on Madagaskar, that had a wingspan of more then 21 feet? I think you know what I am thinking of.
After you have completed your path of involution you have to start the regression: You are invoking the chosen animals down the sequence to the primordial slime, where anything began. Do those various invocations in a certain timespan, for example: You invoke one animal for twenty minutes, then take a break of five minutes, and then go on to the next one. The atavistic states must come to your consciousness and you must understand an atavism, before continuing to the next - as Austin Osman Spare, a western hexenmaster and artist, said: "We do not know the object of perception, but by becoming it." - After arriving at the starting point you can reverse the whole thing, returning to your current place in the evoultionary development. This exercise can take some time as you already may have figured out, but it is up to you to choose the pace of regression and evolution.
H.P. Lovecrafts "The Ancestor" inspired this exercise. You can choose musical accompaniment for the rites of invocation. Start with modern music and end up in simple and primordial sounds. If you do not want to conduct the whole atavistic ritual, you can choose only some of those animals, or only one, and invoke it. This ritual is only one possible way to get into contact with our really old ancestors. Try to find someone who accompany you that remain outside the trance-states you induce. It is better to have someone around when your aviatic atavism chooses to climb up a tree in order to take a short flight around the place. This atavistic exercise is quite similar to the animal-dance. Contrary to the animal-dance you are choosing animals that actually do not have any role in your shamanic acitivities or cosmology. After doing this exercise, this may change, however.