Event 10

1 billion years ago

The Earliest Animal Form? A 2008 NSF study which mapped the evolutionary history of animals argues that the comb jelly, or Ctenophore, may have been the first animal on earth, rather than the sponge. This is a photo of the “Red Tortugas” comb jelly. Comb jellies that dwell in the deep seas are often brilliantly pigmented.
Credit: Photo by R. Griswold, NOAA/Wikipedia

We belong to the kingdom Animalia, better known as the animals. Unlike plants, animals usually move around freely and find and ingest their food for energy (rather than relying on the sun for photosynthesis). Animals are multicellular organisms (having many cells), and are eukaryotes (each cell having a nucleus.) It is thought that the earliest animals, as well as sexual reproduction, first emerged around 1 billion years ago. Tracks and burrows in rocks of this age suggest that some of the earliest multicellular animal life (the Metazoa) were worm-like, and would have been the most complex form of life at the time.


Spongia officinalis
Another study conducted in 2009 indicated that the sponge may afterall be more closely related to the ancestors of all Metazoa, or multicellular animals. This is a photo of Spongia officinalis, or “the kitchen sponge,” which is usually gray when alive.
Credit: Photo by Guido Picchett/Wikipedia.

The first definite fossils of metazoan animal life are only about 600 million years old, and are found in ancient rocks in places like Australia and South Africa (to be discussed in the next section). But it is assumed that the animal kingdom began much earlier, perhaps by a billion years ago. With animals come the first animals that could move around by their own means. Unfortunately these primeval animals left little fossil evidence behind, probably because they were very small creatures and had no hard body parts to be preserved. The rise of more complex forms of plants and animals also heralded the decline of the single-celled bacterial stromatolites which had dominated the earlier fossil record.

The rise of multicellular plants and animals also heralded the origins of sex, where many of these organisms reproduced not simply by dividing into two identical cells (mitosis) but by producing sex cells with one-half of the genetic information, to be merged with a sex cell of an individual of the opposite sex (meiosis or recombination). Sexual reproduction can produce much more variability (unlike simple “cloning” in asexual reproduction). More variability potentially increases the rate of evolution by natural selection.

A likely scenario for the progression of the evolution of animals from early single-celled organisms would be:
1) single-celled amoebae without a nucleus (prokaryotes);
2) single-celled organisms with a nucleus (early eukaryotes);
3) single-celled eukaryotes with flagellae (whip-like tails) to move around;
4) single-celled eukaryotes with mitochondria (organelles, once probably separate bacteria that merged with nuclear cells, to become important structures that produce energy);
5) multicellular organisms (more complex plants and animals).

It has been suggested that the Snowball Earth 2 event (1 billion-700 million years ago) may have caused harsh enough conditions to prevent the evolution of more complex animal forms. With the end of these frozen conditions and a gradual warming of the Earth’s oceans, the first known complex animal forms emerge in the fossil record.


Clear comb jelly
Comb jellies that are found near the ocean surface are often clear or very lightly pigmented.
Credit: Photo by Samuel Blanning, NOAA/Wikipedia.


The transition from single-celled organisms to multi-celled plants and animals is not well-recorded in the fossil record; fortunately, modern organisms exist that can give important clues to this major transition in prehistoric times.

The animals provided life forms a whole new set of opportunities for evolution and change – They can move around, feed on the many types of plants available and also, in many cases, on other animals. Such animals really got an opportunity to take off after the last Snowball Earth period, and have given rise to many different groups over the past several hundred million years – including ourselves!




Astrobiology Magazine article on the earliest animals:

A 2008 article where researchers announce that the comb jelly might be the first animal, not the sponge.

Images from NSF of the first animal on earth, the comb jelly. Posted April 10, 2008.

University of California Museum of Paleontology web page on early “Vendian” animals:


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