What Caused Life’s Major Evolutionary Transitions

Explore the fascinating transition from early cells to multi-celled animals and see the experiments that helped us understand how it happened.


When looking at the fossil record and when comparing different creatures alive today, we see that during the evolutionary history of life, several dramatic jumps in complexity occurred. These jumps are so big that, at least at first glance, they don’t seem to be easily explained with an appeal to evolution as described by Darwin: Descent with modification, acted upon by selection.

Biologists call these “Major Evolutionary Transitions”, 3 of which have been identified in the pathway that eventually gave rise to all animals, along with humankind. Experiments and mathematical models show us that these transitions actually are caused by normal Darwinian evolution, but under specific circumstances. These transitions in individuality occur when natural selection favors cooperation between 2 organisms. They can be related to one another (parent child) or unrelated. Once cooperation is established by natural selection, division of labor often evolves. Over evolutionary time, partners can lose the ability to perform all tasks required to survive and reproduce on their own, thus becoming dependent on the group. When an entire group is interdependent on its cooperating members, a new level of individuality has evolved.

The ability to evolve larger cooperative groups, even between species, has now been directly observed in the lab. The evolution of division of labor has also been demonstrated in the lab. The evolution of full interdependence between group members (thus producing an irreversible jump in individuality) has been demonstrated mathematically but not yet fully observed in the lab.

For Teachers

The content of this video meets criteria in the following Disciplinary Core Ideas defined by Next Generation Science Standards. Use our videos to supplement classroom curriculum.


Our videos benefit from guidance and advice provided by experts in science and education. This animation is the result of collaboration between the following scientists, educators, and our team of creatives.

  • Jon Perry
  • Jeremiah Deasey
  • Anthony Danzl
  • Zaid Ghasib
  • Johana Revel, PhD




  • The paper by Dr Margulis (Sagan) on the origin of mitochondria is called On the Origin of Mitosing Cells.
    Note that she presents several ideas cautiously as hypothesis. While some aspects of the paper are now known to be incorrect, she was spot on about the origin of mitochondria.