Meteorite bombardment may have made Earth more habitable, says study.
Saved by Junk DNA.
The Origin of Life on Earth is a puzzle that biologists, biochemists, physicists and geologists – to name a few – have chewed on over the past 150 years since Darwin opened up the conceptual doors and let in the refreshing light of natural selection. One related question is just when was Earth first inhabited and habitable. The first study above seems to indicate that both Mars and Earth were made more clement by that last gasp of accretion, the Late Heavy Bombardment, which pounded the Inner Planets some 4.0-3.9 billion years ago.
How so? The infalling meteorites released both water and carbon dioxide, thus wetting & warming both planets, perhaps sufficiently for liquid water to remain stable on the open surface. Prior to that event, water may well have been mostly frozen. There’s good reason to think that the process of making long-chains of biomolecules, an important step before ‘Life’ itself, was via concentration of smaller sub-units within ice. Tiny channels of unfrozen liquid become increasingly concentrated in solutes as watery solutions freeze, providing an accelerated reaction environment for polymerisation. In such conditions even quite short pieces of RNA become capable of ‘ligation’, the fusing of RNA sub-units into longer chains.
Once RNA Life has given way to DNA Life what drives the evolution of ever longer strings of DNA and thus ever more complex Life? The second news piece is about evidence that so-called ‘Junk DNA’ – mostly repetitive segments of DNA with no obvious function – actually promotes faster evolution of organisms by altering the rate of gene mutation and gene expression. It seems the ‘Junk’ can make a gene’s DNA sequence more exposed and liable to change when the ‘Junk’ situated next to it has changed in length.
But there’s always a trade off. ‘Junk’ DNA is reduced in some organisms, very noticeably in birds, while it has immensely expanded in some organisms, like certain plants and slow-living creatures like amphibians and lungfish. One’s pace of life style has a distinct selective role on ‘Junk’ DNA’s quantity – fast-living reduces its presence, and perhaps its selective advantage. Birds need to rapidly churn out proteins from their DNA genes and operate at a higher blood temperature too. This might make the DNA more liable to change – birds are immensely speciose – without any ‘Junk’ DNA putting pressure on genes at all. Lungfish, and their kin, live ‘cold-blooded’ rather sedate lives, and carry around a large load of ‘Junk’ that ensures their DNA remains healthy, making the invasion of ‘DNA’ parasites, like viruses, much harder because the host DNA is already full of virus-like ‘Junk’.