A2.1 Origins of cells questions for IB Biology

Why did more ultraviolet (UV) radiation reach the surface of the early Earth than reaches it today?

Researchers repeated a version of the Miller–Urey experiment. Sealed glass flasks containing water and a mixture of gases were heated so that water vapour circulated past a pair of electrodes. Two gas mixtures were tested: a reducing mixture (methane, ammonia and hydrogen) and a neutral mixture (carbon dioxide and nitrogen). Sparks were passed between the electrodes continuously, and the total quantity of amino acids in the water was measured every two days. A further flask containing the reducing mixture was set up identically but with no sparks. Each treatment was carried out three times and the means are shown.

Hypotheses about the nature and origin of genetic material have been tested by experiments.

Which conditions on the early Earth may have allowed carbon compounds to form spontaneously, by processes that do not occur today?

I. High levels of ultraviolet radiation reaching the surface
II. Absence of free oxygen in the atmosphere
III. Low concentrations of carbon dioxide and methane

Fatty acids form micelles in alkaline solution but coalesce into vesicles (spherical bilayers) when the pH is lowered. Vesicles scatter light, so the turbidity (absorbance at 400 nm) of the solution increases as they form. Researchers lowered the pH of a decanoic acid solution to pH 7.5 and measured absorbance every 10 minutes, with and without small particles of montmorillonite, a clay mineral that was common on the early Earth. Absorbance readings were corrected using a tube containing the clay suspension without decanoic acid. Five repeats were carried out for each treatment; error bars show ± one standard deviation.

Several kinds of evidence are used to estimate when life first appeared on Earth. The table shows estimated ages for some of this evidence.