The platypus comes from a very ancient line of mammals; thus the ancestor to all mammals may have had a Z - like sex chromosome at one time. (sciencemag.org)
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Most biologists typically recognize three official branches of life: the eukaryotes, which are organisms whose cells have a nucleus; bacteria, the single - celled organisms that may or may not possess a nucleus; and archaea, an ancient line of microbes without nuclei that may make up as much as a third of all life on Earth (See «Will the Methane Bubble Burst?» (discovermagazine.com)