The Evolution of Ants
Where did ants come from? What were the evolutionary ancestors of ants?
In this article, I discuss the evolution of ants. I answer the question of the origin of ants, their ancestors, and the evolutionary steps that occurred to bring us the ants we know today.
The Origin of Ants
Where, evolutionary speaking, did ants come from? Did they evolve from a common insect ancestor? Did they start as one type of insect, and become ants? Or have ants been ants right from the start?
Prior to 1967, myrmecologists had found no fossils of an extinct species of ant; any fossils that had been found were of species of ants still in existence today. Fossils had been dated back to either the Oligocene Epoch (33.9 million to 23 million years ago), or the Miocene Epoch (23 million to 5.3 million years ago).
However, in 1967, Wilson et al made a remarkable discovery. The first-ever fossil of an extinct species of ant. The fossilised ant originated from the Cretaceous period. This ant, fossilised in amber, was given the name Spechomyrma freyi and was assigned to a new sub-family, Spechomyrminae. Initially, it was estimated to have been about 100 million years old. Myrmecologists revised this date to be some 80 million years.
Sphecomyrma freyi - the origin of ants
This was a much celebrated and important find on the journey of discovering the evolution of ants, the ancestor of the ant. Sphecomyrma freyi proves an almost perfect link between the contemporary living ants and the non-social aculeate wasp. Aculeate refers to insects of the Hymenoptera order in which the ovipositor (the tube-like egg-laying organ) has evolved into a sting.
The ants of the Cretaceous period had some bodily traits that were wasp-like, namely;
(i) a wasp-like pair of mandibles that were noticeably short, with only two teeth
(ii) a gaster that was non-constricted and had an extrusible sting.
(iii) the middle and hind legs had double tibial spurs.
Sphecomyrma freyi has features typically found in ants, namely;
(i) thorax reduced in size and wingless
(ii) petiole (waist) pinched posteriorly at its join with the abdomen.
(iii) the presence of a metapleural gland, being a key trait belonging to ants.
This celebrated fossil discovery was the “missing link” between ants and wasps. It showed a key part of revealing the evolution of ants. Myrmecologists were now confident that ants descended from an early form of solitary wasp.
Wasps become ants - the evolution of ants
Sphecomyrma freyi proved to be the ancestor of our ants. The next question to ask was, how did wasps give rise to ants? Or, how did an early form of wasp branch off the evolutionary tree to start the origin of ants?
It is a fact that all ants are eusocial, meaning that they live cooperatively together in groups, consisting of a small number reproductives (queens and males) amongst a far larger number of non-reproductives (workers and soldiers).
The primitive wasps from which the ant descended were non-eusocial, living solitary lives. There was no worker caste present at this stage.
13 stages of the evolution of eusociality
In 1958, Howard E. Evans, an American entomologist who produced important work on the subject of insect behaviour and evolution, proposed an ethocline, in which he described the progressive change in the pattern of behaviour of closely related organisms. Evans drew on his extensive work and knowledge of solitary Hymenoptera to produce his 13 stages of the evolution of solitary non-eusocial organisms into those of the eusocial order.
Here I summarise the 13 stages proffered by Evans. The summary assumes that the first stage describes a solitary wasp of undetermined species. The final stage represents today’s ant species as a whole.
Stage 1 – A solitary female wasp stings her prey and lays an egg on it, leaving it to fend for itself.
S2 – A solitary female wasp stings her prey and hides it. She lays an egg on it and leaves, not returning.
S3 – Solitary wasp stings her prey, builds a nest at that location, places the prey into it, on which she lays an egg.
S4 – A solitary wasp builds a nest. She looks for food and carries the stung prey to her nest. She lays an egg on, or near the prey, and leaves it to fend for itself.
S5 – The same as stage 4 above. However, the wasp collects more food for her brood and places it into the nest before she leaves for good.
S6 – As stage 5 above, but rather than hoarding prey, the wasp progressively supplies food to her offspring once it reaches its larval form.
Parent remains with brood - the evolution of ants
S7 – The origin of ants continues. As stage 6 above, but the wasp progressively supplies food even before the larvae has emerged from the egg. Here our wasp is no longer laying an egg and abandoning it. Now she stays with her first offspring.
S8 – The wasp now not only supplies food for the larvae, but she cuts up the food and feeds it directly to the larvae rather than just placing the food down for the larvae to feed itself.
S9 – The solitary wasp stays in the nest, as successive generations of her offspring are produced.
S10 – The new adult offspring of our founding mother wasp starts to take care of the brood, feeding the larvae and each other via trophallaxis. The offspring are all reproductives; no worker caste is present yet.
S11 – A worker caste arises, and our mother queen becomes dominant. The worker caste is sterile and unable to produce workers or queens. Males are produced.
S12 – The larvae are fed varying amounts of food. This gives rise to distinctive caste sizes, whereas before the workers were indistinct from the queen.
S13 – A physically distinctive worker class now arises.
The workers forage on the ground for food. The workers lose their wings, a hindrance now.
Others have put forth some proposals to account for the rise of polygyny, the presence of multiple egg-laying queens in the same colony. West-Eberhardin, 1978 is an example. She said that aggregating foundresses typically were sisters or close cousins in the evolution of ants.
I hope this article has been of interest to you, and that I have not made it too complicated. If you have any questions about the evolution of ants and eusociality, or I have made an error in the information I have supplied above, please feel free to contact me. You can comment on this article to let me know, or you can contact me through the contact page. Thank you for reading.
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