Algae seems to appear on the scene between 1.4 billion and 1.2 billion years ago, and algae (specifically red algae) is probably the first multi-celled form of life. It also seems to be the first form of life that reproduced with sexual reproduction, so algae gets credit for inventing sex and probably life-spans (single-celled life might be said to reproduce rather than dying, but sexually reproducing multi-celled life has offspring and eventually dies). Algae comes along about half-way through the Proterozoic Era (2.5 billion years ago until 543 million years ago), and its appearance may contribute to the increase in oxygen in the atmosphere. That is, cyanobacteria created the Oxygen Crisis, but Algae may also have contributed. There are some fossils from about 2.1 billion years ago that might represent multi-celled organisms, and so it’s possible that eukaryotic algae evolved 2.1 billion years ago, rather than merely 1.2-1.4 billion years ago (in which case they helped the cyanobacteria change the atmosphere and kill of many bacterial life forms in the Oxygen Crisis).

If life appeared 4.1 billion years ago, it seems for more than the first half of all the time there has been life, that life was single-celled or colonial life. Multi-celled life has only existed for the second half of of the time span that life has existed on earth.

The Red Algae and Brown Algae came first. They contain phycobilin pigments and some different varieties of chlorophyll. Green Algae seem to have evolved about 475 million years ago, and they just use chlorophyll without the phycobilin pigments. Algae come in many different forms: some are single-celled, others live in colonies of single cells, some are multicellular, and others live in symbiosis with other organisms, most notably some algae live with fungi to create lichens. Some Algae move around with flagella or else by moving along like amoebas. Green Algae are the ancestors of the mosses and other primitive land plants.

The Endosymbiotic transfer theory suggests that some primitive multicellular life form ancestral to animals, fungi, protoctista, and plants was created when an ancestral eukaryote merged with an eubacteria (probably a purple bacteria, which became the mitochondria in non-bacterial and non-Archean life). This original primitive Eukaryote life with the mitochondria organelles then branched in two directions, with the common ancestor of red algae, brown algae, and green algae (and, through green algae, all plants) coming into existence by the merger of a phototrophic bacteria (another Eubacteria) merging with the Eukaryote and becoming the chloroplast organelles we find in algae and plants. In other words, the cyanobacteria, which were formerly called Blue-Green Algae didn’t evolve into algae, but rather, they became chloroplasts in algae by merging with some primitive Eukaryotes.

This scum appears to be a bloom of filamentous green algae, probably Spirogyra.
However, bright green foams like this may also indicate an Euglena scum. Euglena is a flagellated algae.

This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License .