BY DR. JOSE PALU-AY DACUDAO
MOST biologists would say that a thing is deemed a living organism if it metabolizes, reproduces, and mutates.
There is a controversy if viruses are living organisms because viruses intrinsically do not metabolize, but use their hosts’ metabolic pathways to produce more viruses.
With the advent of microscopes, it was discovered that microbes could roughly be divided into the anatomically simple ones that lacked cell organelles, and more complex ones that possessed cell organelles.
Fast forward to the late 1930s, the French biologist Edouard Chatton divided microbes on this basis, with those that lacked cell organelles falling under the biological empire prokaryote.
In the early 1960s, Canadian bacteriologist Roger Yates Stanier and Cornelis Bernardus van Niel classified not only complex one-celled organisms, but also all multicellular life forms (meaning multicellular algae, fungi, animals, and plants), as eukaryotes, because multicellular organisms have cells that possess organelles, in a two-kingdom (or two-super-kingdom) classification).
All biologists today accept the idea that all living organisms are either prokaryote or eukaryote.
Here’s a recap of the previous article:
As microscopes, phylogenetics, comparative analysis proteins and of genes and gene sequence among different organisms, and cladistics improved over the decades, the highest classification for living organisms for a long time was successfully changed. It was realized that the classification of “animals” and “plants” was severely lacking.
Some traditional mobile one-celled animals could both eat other microbes and photosynthesize. Photosynthetic organisms had close cousins that had lost the ability to photosynthesize. And weirdly enough, it was discovered that mitochondria and some other cell organelles had structures which made them resemble bacteria.
So nowadays, living organisms are broadly divided into prokaryotes and eukaryotes. Prokaryotes are one-celled organisms that lack a nucleus, organelles, and other specialized cell structures. Eukaryotes have these.
We humans are eukaryotes, as are all traditional “animals” and almost all of traditional “plants”.
It has been theorized since the 1960s that these organelles are actually bacteria and cyanobacteria, that is prokaryotes, that got incorporated into now eukaryotic cell.
In brief, once upon a time, prokaryotes ingested prokaryotes, and instead of the swallowed prokaryotes getting digested, they survived and formed symbiotic relationships with the prokaryotes that ingested them.
Ancient bacteria that could do oxidative phosphorylation became mitochondria, the eukaryotic cell organelle that ultimately oxidizes our food. Cyanobacteria, which are photosynthetic bacteria, became the chloroplasts of eukaryotic “green plants”.
In the late 1970s, Carl Woese and George E. Fox, after intensive studies on 16S ribosomal RNA (16S rRNA) genes, found a fundamental difference within traditional bacteria. Subsequently, they further classified prokaryotes into two kingdoms, archaebacteria (archaea) and eubacteria (or just bacteria).
Archaea are by no means insignificant in ecosystems. For instance, all microbes that can do methanogenesis (produce methane) are archaea. Thus, Woese and Fox divided living organisms into three- domain system – bacteria, archaea, and eukarya (eukaryotes that have complex cells with organelles). We humans and all multicellular organisms are eukaryotes.
There remains the question: which prokaryote phagocytosed which prokaryote, thus creating eukaryotes?
As mentioned above, photosynthetic cyanobacteria evolved into the chloroplasts of photosynthetic eukaryotic plants.
What kind of prokaryote ingested the cyanobacteria? Another bacteria? Or an archaea? And how about mitochondria? Where they originally bacteria or archaea?
The discovery of new archaea microbes and intensive studies in the first two decades of the 21st century on ribosome protein sequencing, cellular proteins, and gene sequences and sets have led biologists to conclude that an ancient microbe belonging to the domain archaea phagocytized another microbe belonging to the domain bacteria, and subsequently evolved into the first eukaryote.
That would make all members of domain eukarya essentially archaea that ingested various bacteria, and formed symbiotic relationships with them. Thus, under the two -domain system of classification, eukarya is subsumed under archaea.
In other words, in the two-domain system, there is archaea and bacteria, and all eukaryotes are archaeans (whose cells contain symbiotic bacteria).
Weirdly enough, we humans are archaea, more related to methane-producing archaean microbes than to traditional bacteria./PN
