Wednesday, 4 May 2022

Evolution News - How the Black Rat's Environment was Provided By Humans

Black rats, Rattus rattus

Source: Wikipedia by Kilesson - own work. CC BY-SA 3.0
how the black rat colonised Europe in the Roman and Medieval periods - News and events, University of York

An open access paper published in Nature Communications yesterday, will be understood by anyone with even a basic understanding of evolutionary biology as an example of evolution in progress, but will probably be waved aside by creationists because the subject of it, the black rat, Rattus rattus, didn't change into a cat or a dog or even a new species.

The paper records how the black rat colonised Europe on two separate occasions, separated by a dramatic decline and extinction in some areas. Each rise and fall in population and each wave of expansion was facilitated by patterns of human habitation and trade because the species had evolved to benefit from just those human activities as a commensal species on human civilisation.

We’ve long known that the spread of rats is linked to human events, and we suspected that Roman expansion brought them north into Europe.

But one remarkable result of our study is quite how much of a single event this seems to have been: all of our Roman rat bones from England to Serbia form a single group in genetic terms.

When rats reappear in the Medieval period we see a completely different genetic signature – but again all of our samples from England to Hungary to Finland all group together. We couldn’t have hoped for clearer evidence of repeated colonisation of Europe.

Our results show how human-commensal species like the black rat, animals which flourish around human settlements, can act as ideal proxies for human historical processes.

Dr David Orton, co-author
Department of Archaeology
University of York, York, UK.

The modern dominance of brown rats has obscured the fascinating history of black rats in Europe. Generating genetic signatures of these ancient black rats reveals how closely black rat and human population dynamics mirror each other.

Alexandra Jamieson, co-first author
Palaeogenomics & Bio-Archaeology Research Network
Research Laboratory for Archaeology and History of Art
University of Oxford, Oxford, UK
The study was conducted by a team led by scientists from the University of York along with colleagues from the University of Oxford and the Max Planck Institute, who conducted DNA analysis of archaeological remains. These showed that the rats expanded into Europe in two waves, each with a characteristic DNA signature, showing that they originated from two different populations.

This study is a great showcase of how the genetic background of human commensal species, like the black rat, could reflect historical or economic events. And more attention should be paid to these often neglected small animals.

He Yu, co-first author
Department of Archaeogenetics
Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany And School of Life Sciences
Peking University
Beijing, China
Archaeological black rat mandible.

Image credit: Ewan Chipping, University of York
The first wave occurred during the Roman Empire when the 'Pax Romana' created the stable conditions for population growth and trade with the black rat able to travel along trade routes and exploit human settlements, crops and refuse. With the collapse of the Roman Empire in the Early Middle Ages, the economy collapsed, the population fell, and along with it, the black rat population. This could have been precipitated by the 'Justinian Plague', an early form of the Black Death, caused by the bacterium, Yersinia pestis which killed about 20% of the population of Constantinople and affected the entire Mediterranean basin, Europe and the Near East.

The second wave came on the back of growth in long range trade in the Middle Ages and the growth of cities as centres of commerce. They have now declined again, this time probably because of competition from the closely related brown rat, Rattus norvegicus, which similarly evolved to be a commensal species on human civilisation, along with the house mouse, Mus musculus and is now the dominant rat in temperate Europe.

Copyright: © 2022 The authors. Published by Springer Nature, Ltd.
Open access
The team's paper was published open access yesterday in the journal Nature Communications:
Abstract

The distribution of the black rat (Rattus rattus) has been heavily influenced by its association with humans. The dispersal history of this non-native commensal rodent across Europe, however, remains poorly understood, and different introductions may have occurred during the Roman and medieval periods. Here, in order to reconstruct the population history of European black rats, we first generate a de novo genome assembly of the black rat. We then sequence 67 ancient and three modern black rat mitogenomes, and 36 ancient and three modern nuclear genomes from archaeological sites spanning the 1st-17th centuries CE in Europe and North Africa. Analyses of our newly reported sequences, together with published mitochondrial DNA sequences, confirm that black rats were introduced into the Mediterranean and Europe from Southwest Asia. Genomic analyses of the ancient rats reveal a population turnover in temperate Europe between the 6th and 10th centuries CE, coincident with an archaeologically attested decline in the black rat population. The near disappearance and re-emergence of black rats in Europe may have been the result of the breakdown of the Roman Empire, the First Plague Pandemic, and/or post-Roman climatic cooling.
A Population dynamics of the black rat (R. rattus), Asian house rat (R. tanezumi) and brown rat (R. norvegicus) estimated by PSMC, with 100 bootstrap replicates. B Demographic modelling of the divergence and migration among the black rat, Asian house rat and brown rat estimated by G-PhoCS. The values represent the average estimates of effective population sizes (in thousands), population divergence times (Mya) and the total migration rate through time. The 95% HPD range of all estimates are listed in Supplementary Data.

What is probably lost on creationists with their child-like understanding of evolution where one species changes into another, often by giving birth to it, or by changing physically into it like one of their transformer toys, is that findings such as this example of co-evolution where one species' evolutionary path is dependent on that of another species with which it has formed an alliance, is an illustration of how evolution works. Species do not normally evolve significantly in isolation from the other organisms in their ecosystem but move into and occupy niches as and when these become available, often, as in the case of the black rat, created by another organism.

The first major step in this process was of course, the evolution of the black rat to be able to occupy the niche humans were creating in their buildings, food stores, refuse and trade routes. Then, as these developed in Europe the black rat expanded into this newly available range. When that collapsed, the rats lost their habitat and their population crashed along with the human economy. And so the black rat's evolutionary future became dependent on human civilisation and economic activity, to expand its range again as the economy in Europe recovered to its previous level under the Roman Empire, during the Middle Ages, only to lose out again when the brown rat expanded its range in response to the opportunities human civilisation had provided.

In other words, a very nice illustration of evolution in progress, albeit one which creationists wouldn’t recognise or understand because they have a simplistic understanding of what evolution is.

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