From tiny to T-Rex: Why life on Earth got so big
Organisms first grew big to increase their radius of spreading their offspring rather than to compete for food, according to a new study at the University of Cambridge.
4 billion years ago, life on Earth looked nothing like it does today. In fact, the oceans contained only single-celled microbes. At some point, these single-celled organisms began to work together, forming complex, multi-celled creatures such as animals. So why did life suddenly start to become much bigger?
A recent study published in Nature Ecology and Evolution might have the answer. Emily Mitchell at the University of Cambridge studied 635 million year old fossils to understand why life transitioned from tiny microbes to the multicellular life forms we know today.
The fossils (pictured above) were discovered in Newfoundland, Canada, and date back to the Ediacaran time period: a time when organisms did not have mouths, organs, or the means to move. “These [Ediacarans] are the first large things. They are almost certainly our oldest ancestors,” says Mitchell.
These fossils show evidence of fern-like organisms that lived in the ocean. They shot up to about 2 metres tall with stem-like structures, yet surprisingly they were not plants.
“They look incredibly like plants in that they have this strange branching pattern called fractal branching. But these fossils are actually found in deep-sea rocks, so they couldn't have been photosynthetic,” explains Mitchell, “We’re very sure that they didn’t have any light, so they couldn’t have been plants. Some of them may have actually been the first animals.”
The Ediacaran ‘animals’ were fixed in place, deep in the ocean. They sustained themselves by absorbing nutrients in the surrounding water. Initially, researchers believed these organisms evolved and grew quickly to compete for food due to their stationary states. It was thought that the taller these organisms grew, the more nutrients they could obtain. However, Mitchell and her co-author, Charlotte Kenchington, did not find this to be true.
“I looked at the positions of different species of fossils on the rocks. If two different species are competing a lot together, they tend to die and get spaced out. However, we did not find that this was the case,” says Mitchell.
The ocean of the Ediacaran time period was an all you can eat buffet; the water was rich with nutrients and therefore organisms did not need to compete for food. If the ediacaran organisms were not competing, why did they get so tall?
The answer lies in how Ediacaran organisms reproduced. Many of these organisms could reproduce in two ways. They were capable of cloning themselves by the use of stolons, which is common in strawberry plants. Stolons are horizontal stems that originate from the parent organism, and take root to create an entirely new organism. Ediacarans could also shoot out their reproductive material (e.g. eggs and sperm) into the surrounding water, similarly to jellyfish. So what does height have to do with it?
Through spatial analysis, Mitchell was able to create a map of the Ediacaran organisms and their offspring. The locations of the offspring were concentrated in groupings, called dispersal clusters.
“The taller the Ediacaran organism was, the bigger their dispersal cluster was,” explains Mitchell, “this shows that the big advantage of being large was that they could disperse their offspring further; it was reproduction that was driving the large body size, not competition for food.”
In this case, bigger really was better!