What is the Last Universal Common Ancestor (LUCA)?

About LUCA:

• LUCA is the hypothesized common ancestor from which all modern cellular life, from single-celled organisms like bacteria to the gigantic redwood trees (as well as us humans), descends. 
• LUCA represents the root of the tree of life before it splits into the groups recognized today: Bacteria, Archaea, and Eukarya.
• Modern life evolved from LUCA from various different sources: the same amino acids used to build proteins in all cellular organisms, the shared energy currency (ATP), the presence of cellular machinery like the ribosome and others associated with making proteins from the information stored in DNA, and even the fact that all cellular life uses DNA itself as a way of storing information.
• Through genetic analysis and evolutionary modeling, researchers pinpointed LUCA’s existence to about 4.2 billion years ago, just 400 million years after the formation of Earth and our Solar System.
• LUCA is not too different from modern prokaryotes and clearly possessed an early immune system.