If you want good evidence of the common origins of living organisms on Earth, it would be difficult to find anything better than the tardigrades or water bears.
These strange little creatures live just about everywhere on Earth from mountain tops to ocean depths, from hot springs to polar ice. They will even be found living in your guttering, and they are almost indestructible. They are so different to other creatures that they are given a phylum all of their own, the Tardigrada. There are something like 1,150 different species.
They survive long periods of extremely inhospitable conditions by withdrawing all their appendages inside their bodies to reduce their surface area to volume ratio, shed as much water as possible becoming practically dehiscent, and shutting down their metabolism. Basically, they dry up and become what scientists call 'tuns', in a state of cryptobiosis.
As Cornelia Dean said in a recent New York Times article:
Confronted with drying, rapid temperature changes, changes in water salinity or other problems, tardigrades can curtail their metabolism to 0.01 percent of normal, entering a kind of suspended animation in which they lose “the vast, vast, vast majority of their body water,” Dr. Siddall said. They curl up into something called a “tun.”
Tuns can be subjected to atmospheric pressure 600 times that of the surface of Earth, and they will bounce right back. They can be chilled to more than 300 degrees Fahrenheit below zero for more than a year, no problem. The European Space Agency once sent tuns into space: Two-thirds survived simultaneous exposure to solar radiation and the vacuum of space.
Without water, “the damaging effects of freezing cannot happen,” Dr. Siddall explained. “It protects against heat because the water inside cannot turn into a gas that expands.” Even radiation needs water to do damage, he said. When cosmic radiation hits water in a cell, it produces a highly reactive form of oxygen that damages cell DNA. The tun doesn’t have this problem.
That's right, tardigrades are literally part animal, part plant, part bacteria and part fungi! Approximately one sixth of their genome is from other only very distantly related organisms. Many of the acquired genes appear to be from organisms which are themselves adapted to extreme conditions, which is hardly surprising since they live alongside these organisms.
Despite fascinating scientists for over 200 years, little at the molecular level is known about tardigrades, microscopic animals resistant to extreme stresses. We present the genome of a tardigrade. Approximately one-sixth of the genes in the tardigrade genome were found to have been acquired through horizontal transfer, a proportion nearly double the proportion of previous known cases of extreme horizontal gene transfer (HGT) in animals. Foreign genes have impacted the composition of the tardigrade genome: supplementing, expanding, and replacing endogenous gene families, including those families implicated in stress tolerance. Our results extend recent findings that HGT is more prevalent in animals than previously suspected, and they suggest that organisms that survive extreme stresses might be predisposed to acquiring foreign genes.
Horizontal gene transfer (HGT), or the transfer of genes between species, has been recognized recently as more pervasive than previously suspected. Here, we report evidence for an unprecedented degree of HGT into an animal genome, based on a draft genome of a tardigrade, Hypsibius dujardini. Tardigrades are microscopic eight-legged animals that are famous for their ability to survive extreme conditions. Genome sequencing, direct confirmation of physical linkage, and phylogenetic analysis revealed that a large fraction of the H. dujardini genome is derived from diverse bacteria as well as plants, fungi, and Archaea. We estimate that approximately one-sixth of tardigrade genes entered by HGT, nearly double the fraction found in the most extreme cases of HGT into animals known to date. Foreign genes have supplemented, expanded, and even replaced some metazoan gene families within the tardigrade genome. Our results demonstrate that an unexpectedly large fraction of an animal genome can be derived from foreign sources. We speculate that animals that can survive extremes may be particularly prone to acquiring foreign genes.*
Thomas C. Boothby, Jennifer R. Tenlen, Frank W. Smith, Jeremy R. Wang, Kiera A. Patanella, Erin Osborne Nishimura, Sophia C. Tintori, Qing Li, Corbin D. Jones, Mark Yandell, David N. Messina, Jarret Glasscock, and Bob Goldstein
Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade
PNAS 2015 : 1510461112v1-201510461. doi: 10.1073/pnas.1510461112
*Copyright © Boothby, T.C., et al / National Academy of Sciences.
During their long periods as tuns, the DNA of tardigrades is liable to get damaged and broken into fragments so it has to be repaired and reconstructed when conditions improve and the tuns rehydrate. During this process, other bits of DNA can occasionally be incorporated into the repaired genome. In the event that this conveys some benefit, the lucky recipient will be more successful and leave more descendants, so the newly acquired genes will spread in the genepool over time, just as they would had they been produced by mutation.
So, tardigrades seem to have found a shortcut to evolution, acquiring the genes which have evolved and been been selected for fitness in these extreme conditions in other organisms.
The significance of this discovery, apart from is showing that, in certain conditions, horizontal gene transfer might play a bigger role than was previously appreciated, is that is shows the basic gene-centred nature of evolution and the common origin of the genetic code. A gene for basic physiological functions can work just as well in one order as in another. A gene has no concerns about whether it finds itself in a plant, a fungus, a bacterium or a tardigrade cell. If it works well in alliance with the other genes it happens to find itself in in the same organism then it will live long and prosper, as will the modified organism in which this alliance of genes exists.
This is only possible because the way DNA is transcribed into RNA, the way RNA is translated into a sequence of amino acids to make a protein, and the way DNA is replicated, is identical for all life because all life has evolved out of the original organisms in which this data storage, retrieval and processing system originated.
There is probably no better illustration of how living systems are all related than this ability of genes to be horizontally transferred between organisms on widely separated branches of the tree of life, and to continue to function as though nothing had happened. It also neatly illustrates the fact that our human classification system is essentially artificial and nature has no obligation to follow the rules of taxonomy.
So when unfortunate creationist dupes chant their protective mantras about 'micro' and 'macro' evolution being somehow materially different things, they simply show the world how their ignorance of biology makes them such easy victims of the frauds who pose as creation 'scientists'.
Thanks to Facebook friend, Bill Wight, for pointing me to this research.
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