Bears. Donkeys. Fat, friendly dogs. These animals — animals, generally — have been around for an extremely long time, long enough to feel like a fixed part of the landscape. It’s easy to forget that these creatures weren’t always there, and didn’t always look like they do now. On human—as opposed to geologic — time, forms seem more or less fixed; sexual mores and national attitudes towards fascism might change in the course of one’s lifetime, but zebras stay more or less the same.
Taking the long view, though, it’s worth wondering — which of these animals, as we know them now, has been around for the least amount of time?
For this week’s The Extreme Life of the Sea, among other books
A lot of species “appear” earlier to each other than they do to us—they are called cryptic species, and they are good species by the main definitions, in that they do not interbreed with even the most similar species around them. This basically means that the species themselves can tell each other apart quite well, so that they do not interbreed. But they are not easily distinguished by us from their sister species by any but the most detailed look. And because they are so similar in so many ways, they can’t really be found in the fossil record, and their date of ‘appearance’ is often only estimated by genetics.
My favourite example right now is a group of magnificent table top corals in the Pacific. They are part of a species complex that all used to be called Acropora hyacinthus. But there are at least six cryptic species in this complex—up to four of them living together on reefs like in American Samoa, the Cook Islands, Palau, the GBR, etc. The reason this is my favourite is that one of these species has evolved traits that we are finding very useful right now: they are resistant to the heat waves caused by climate change sweeping across the tropics and killing corals wholesale.
This species, still only named by a letter designation He (distinguished from Ha, Hb, Hc, Hd, Hf) is the most heat resistant of the group, associates with the most heat resistant symbionts, and lives in the warmest microclimates. When we moved it to a nearby reef and grew it in coral nurseries, the nubbins performed better and survived heat waves more. This species complex is more ready for future climate change than we would know if we thought all corals with the same latin name (Acropora hyacinthus) were all the same.
Tim Barraclough
Professor, Evolutionary Biology, Imperial College London, who works on the evolution of species diversity
Humans are discovering new species all of the time, but most of those have been around for a long time. Either we only just found them or we only just realised they are different enough from related animals to justify the name of a new species. The question of which animal species formed most recently is quite hard to answer because the process of divergence into separate species — called speciation — takes place over long time periods and therefore it is hard to categorically say at what point there is a transition from “two populations of the same species” to “two species”. We can, however, estimate the divergence time between separate species using molecular clock approaches: the tendency for DNA sequence differences to accumulate at a roughly constant rate over time.
Caveats out of the way, we can attempt to answer the question. One of the most spectacularly rapid radiations of animal species is cichlid fish of Lake Victoria, in which over 500 species have originated from a single common ancestor between about 100,000 to 400,000 years ago. We cannot easily date the origin of each species because there has not been enough time to accumulate DNA sequence differences, but with a few assumptions it can be calculated that around 5% of the species would be less than 2000 years old and the youngest species would be around 150 years old. Unfortunately, which cichlid species that is, no-one knows.
An alternative place to look is for species associated with human habitats. For example, the body louse (Pediculus humanus humanus) diverged from the head louse (Pediculus humanus capitis) around 100,000 years ago into a lifestyle associated with the origin of clothing. They are currently classified as sub-species, rather than a full species. Species are also adapting to human-modified habitats, such as cities or farmland. For example, two forms of the malaria-transmitting mosquito Anopheles gambiae have adapted to rice paddy fields versus puddles in West Africa, and both of them are strongly associated with humans.
These have recently been reclassified as Anopholes coluzzii and Anopholes gambiae. Their dependency on high densities of humans would argue for divergence perhaps 5000 years ago, but the divergence time estimated from DNA sequence data is around 60,000 years. Either way, this divergence is very recent compared to most other species. Other cases such as the proposed new species of Culex mosquitos adapted to life in the London Underground system remain ambiguous as these may be older and have colonised from other habitats (e.g. caves).
Speciation is ongoing, and species that are in the process of forming will only become unambiguously separate after longer time has elapsed than we can observe directly. There are many cases of animal species adapting to the impact of humans on global habitats and environment. Whether these populations persist and diverge into full species will depend in part on how long our own footprint lasts on the planet.
Nick Barton
Professor of Evolutionary Biology and Dean of the Graduate School at the Institute of Science and Technology in Austria
Well, this is not so easy to answer: organisms evolve gradually, and so it might not be so easy to find when an animal species reached its “present form.” Also, evidence could only come from fossils, which are patchily distributed. For example, though there is a consensus that “anatomically modern humans” emerged roughly 200,000 years ago, this term is hard to define—traits such as large brains, tool use, and bipedalism evolved at different times across multiple taxa. Perhaps the clearest example would be domesticated animals, such as dogs: morphologically, dogs show as much diversity as typical mammal genera (related groups of species), and most of this diversity arose from organised selection of pedigree breeds in the 19th century.
Sean Mullen
Associate Professor, Biology, Boston University, whose research is aimed at understanding the origin and maintenance of diversity at all levels of biological organisation
This is a pretty difficult question to answer for several reasons. First of all, animal species are constantly evolving, including humans. Second, our knowledge of biodiversity is highly incomplete, so it would be impossible to say for certain whether one species is “the youngest” or “newest”. Third, definitions of what species are vary considerable among scientists and across different groups of organisms… further mudding the waters.
What we do know, as evolutionary biologists, is that there are intrinsic features of organisms that increase their likelihood of rapid speciation. These include things like fast generation times, high mutation rates, etc. In addition, we often see the highest rates of speciation and divergence among groups of organisms that have evolved key evolutionary innovations that allow them to colonize previously unexploited ecological niches. For example, African rift lake cichlid fishes, Darwin’s finches, or neotropical Adelpha butterflies.
The one thing I do really like about this question, however, is that if you asked 100 evolutionary biologists, you’d likely get 100 different and equally wonderful answers!