THE INFORMATION CONTENT OF DNA AND THE ORIGIN OF LIFE

The information content of DNA and the Origin of life

V.V.Morariu

National Institute for Research and Development of Isotopic and Molecular Technologies, P.O.Box 700, 3400 Cluj-Napoca 5, Romania, e-mail:

The first form of terrestrial life is estimated to be 3.5-3.85 billion years old. The younger age evidence for life was fossilized microbes from a rock formation in Australia while the older one, from the carbon isotopes in a ratio characteristic of living organisms in rocks from an island off the coast of Greenland.

Evolution led to new species, which, in time, gradually increased in complexity. This is quantified in the increasing molecular weight of the species DNA. Delsemme converted the molecular weight in quantity of information and constructed an information vs. time linear plot to be referred as Delsemme plot. This illustrates how the information content of the DNA increased in time for the main classes of species. I have reconstructed Delsemme’s plot and represented it on a double logarithmic diagram (fig.1).

Fig.1 Reconstructed Delsemme’s plot. It illustrates the increase of DNA information during evolution. Some of the species are mentioned on the plot. Time is expressed in years by present.

Some contradictions or unclear aspects can be noted in Delsemme’s plot. The hypothetic virus or the protobion, according to Delsemme, represents an emergent form of life before the first bacteria. Although it is specifically mentioned as being a hypothetic primitive form of life, something alike to a virus, the reason for including it on the plot is not clear. Also it is not at all clear why its age is set at 3 billion years by present (b.p.). Further the apparition of life is mentioned on the same plot at 3.8 billion B.P. and it corresponds to information of one word (100 bits). This would have been an even simpler form of virus than the hypothetic virus or protobion. Again this aspect is not discussed in the original paper. However it contradicts the finding of fossilized microbes as a simple form of virus could not leave such fossilized traces. Also it is doubtful that the original form of life could have existed with such a low amount of information.

The younger species can be connected with a straight line in Delsemme’s plot while the older ages clearly show that evolution of life must have proceeded with a much greater speed, as shown by the straight line with the higher slope (fig.1). In fact there is a gap between the apparition of life and the anaerobic bacteria where the actual forms of life remain unknown.

Another problem is to whether a break exists between the prebiotic evolution and post biotic evolution as far as the DNA information content is concerned. We may reasonably suppose that the first form of life must have evolved on a background of existing molecules and structures and they did not differ too much (the protocell and the environment respectively). In other words the information content of nucleic acids during prebiotic and the first forms of life must have been within a close range. Again, the information during the prebiotic evolution of nucleic acids was not considered in the original Delsemme’s paper.

Fig.2 Linear extrapolation of Delsemme’s double logarithmic plot.

A simple linear extrapolation of Delsemme’s double logarithmic plot to older ages goes to nowhere. Although we do not know the information content of the DNA in the first form of life on Earth and what was its prebiotic evolution we must turn to the lowest possible information of the earliest molecules. The first significant information was one bit. We can see on the plot (fig.2) that the linear extrapolation goes behind 100 billions of years when reaching one bit of information. This is meaningless when the age of the Earth is around 5 billion of years and the age of the Universe is established within the range of 12.53 and 15.64.6 billion years (2). The conclusion is that the evolution of DNA must have not been linear (on the double log plot). In mathematical terms such a hypothetic linear plot is equivalent to a fractal rule. However Nature is full of multifractal structures, or in other words, of a nonlinear plot. This is why we performed a polynomial fitting of the data available from Delsemme’s plot. There is no much difference between the linear fitting and the polynomial fitting within the limited range of the known evolution period. However it makes a significant difference when further extrapolating the polynomial curve to older ages. This is illustrated in fig.3. We can notice that the minimum information of 1 bit is attained

Fig. 3 Polynomial fitting of the available evolution data (continuous curve) and extrapolation (isolated data) of Delsemme’s plot. Very low information could have characterized the early stages of the molecular evolution before the smallest DNA molecule appeared. It shows that an information content of 1 bit occurs at about the time of the Big Bang. The human DNA information could be roughly positioned at about 10 million years b.p. The coincidence of the minimum information content (1 bit) with the origin of the Universe may suggest that the

evolution of the terrestrial life is an integral part of the evolution of life in Universe. This does not mean that the origin of information starts exactly at the time of Big Bang. Perhaps it starts at a later date but the precision of the extrapolation at such an old age cannot tell more than that. The consequences of these findings are:

First, it shows that it is plausible to think of the evolution of the terrestrial life as an integral part of the evolution in the Universe.

Second, it links the prebiotic evolution (low information content) to the post-biotic stages as a continuous process. In other words, in terms of the information content, the apparition of life does not represent a sort of a “Big Bang”, at least according to the precision of this estimation. One could possibly argue that the first form of life was RNA and not DNA based (3). However this does not exclude the parallel evolution of DNA.

Third, it tells that the life is of extraterrestrial origin. This implies that life must have been seeded on Earth and this is in agreement with various ideas of panspermia (4).

Fourth, it suggests that at present the evolution of DNA is almost slowed down to an end, and therefore little changes are expected in the future. However we know that even very small differences of DNA make a significant difference in the species such as between a monkey and a human being.

Fifth, it predicts that any other forms of extraterrestrial life must follow a similar course. This does not mean that similar species might exist outside Earth. All what can be said is they could have similar DNA information content regardless of the place in the Universe.

Finally it should be mentioned that the present analysis and conclusions are to be regarded with great caution while they can be the basis for for further discussions on the subject. Nevertheless they represent a coherent picture in agreement with other findings and ideas mentioned in the literature (5-6).

References

  1. A. H. D e l s e m m e, Les cometes et l’origine de la vie, 5eReunion Regionale Europeenne d’Astronomie, Liege, 28 juillet-1er aout, 1980, publiee dans “Variability in Stars and Galaxies”
  2. Age of the Universe,
  3. J.F.Joyce, The antiquity of RNA-based evolution, Nature, 2002 (No.6894) 214- 221.
  4. Cosmic Ancestry, http:/
  5. P. R e a n e y, Scientists Claim Evidence of Life in Outer Space, Science Reuters, 2001 July 31,
  6. N. R. P a c e, The universal nature of biochemistry, Proc. Natl. Acad. Sci. USA, Vol. 98, Issue 3, 805-808, January 30, 2001