For more than a century, it was believed that the simple sponge was the first creature to evolve. However, DNA studies on the invasive comb jelly, Mnemiopsis leidyi, have revealed new evidence to challenge this theory.
Comb jellies (ctenophores) are delicate gelatinous marine animals, similar in appearance to their jellyfish cousins, but without the ability to sting. Also known as sea walnuts, they are native to the coastal waters of the western Atlantic Ocean, but have recently been detected in the North Sea and English Channel.
M. leidyi is a particularly problematic invasive species due to its voracious appetite for fish eggs, larvae and other planktonic organisms. During the 1980s it caused the catastrophic decline of fish and shellfish stocks in the Black and Caspian Seas, resulting in huge economic and ecological damage.
Scientists have long believed that complex animal cells like muscles and nerves only evolved once. However, new evidence has emerged to challenge this theory. Over the last 30 years, scientists have studied the DNA of different organisms to improve our understanding of evolution.
The chemical components of DNA are known as bases and are always found in pairs. The DNA of comb jellies has 150 million base pairs, whilst human DNA has 3 billion. The combination of base pairs within an organism makes up what we call a genome – all the information needed to build and maintain that organism. The genome is divided into sections of DNA known as genes. Each gene contains a set of instructions for a particular function, such as building different types of cells.
DNA analysis of M. leidyi shows that comb jellies branched out from the rest of the animals before the sponge. This suggests they were the first creatures to evolve over 600 million years ago – they are now believed to be the oldest known multicellular organisms, knocking sponges off their evolutionary top spot.
Sponges are very simple animals without any complex cells. Comb jellies are much more complicated. They possess some of the genes present in other animals which are needed for a nervous system, and as a result have a very basic network of nerves. Sponges have these same limited genes, but lack any kind of nervous system – indicating they had the capacity to build one but lost it over time.
Whilst comb jellies also have muscle cells, they do not have the genes responsible for them in other animals. This indicates that muscle cells evolved independently in comb jellies, after they branched off from the other animals.
If comb jellies appeared before sponges, this indicates that early organisms possessed the genes they needed for complex cells, but these were lost over time, or they re-evolved on multiple occasions.
This new information provides a fascinating insight into the events leading to the spectacular diversity seen today in the animal kingdom. Expanding our knowledge of evolution gives us new understanding of genes and their functions – including those in the human genome. Mapping the DNA of an invasive species may one day lead to curing human genetic diseases.
Moroz, L. L., Kocot, K. M., Citarella, M. R., Dosung, S., Norekian, T. P., Povolotskaya, I. S., … & Kohn, A. B. (2014). The ctenophore genome and the evolutionary origins of neural systems. Nature, 510 (7503), 109-114.