ancestor: A predecessor. It could be a family forebear, such as a parent, grandparent or great-great-great grandparent. Or it could be a species, genus, family or other order of organisms from which some later one evolved. For instance, ancient dinosaurs are the ancestors of today’s birds. (antonym: descendant) archaea: (singular: archaeon)  One of the three domains of life on Earth. This group consists of single-celled prokaryotes — organisms without a cell nucleus. Archaea are best known for living in extremely harsh environments, such as very salty water or highly acidic or hot places. bacteria: (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth. biologist: A scientist involved in the study of living things. class: (in taxonomy) A ranking used to group similar organisms. It comes below phylum and just above order in the hierarchy (which ranges in descending order from domain to kingdom, phylum, class, order, family, genus and finally …