2.2: The cell theory and the continuity of life

Toward the end of the 1800’s, observations using microscopes suggested that all organisms contained structurally similar units, termed “cells.” Based on such observations, a rather sweeping conclusion, the Cell Theory, was formulated. The Cell Theory has two distinct parts. The first is the prediction that every organism is composed of one or more, and in some cases millions to billions, of cells together with secreted products, such as bone, hair, scales, and slime. The Cell Theory postulates that cells are membrane-bounded, open (in a controlled way), non-equilibrium systems. Up to the present day) there has been no evidence that modern cells can form from non-cellular materials. Therefore the second part of the Cell Theory is that cells arise only from pre-existing cells. The implication is that organisms, and the cells that form them, arise from pre-existing cells. Cell Theory says nothing about how the first cell or how life on Earth appeared.

Cells contain hereditary information, stored in a physical and relatively stable form, in molecules of double-stranded deoxyribonucleic acid (DNA). The Cell Theory implies that all organisms that exist now and in the past are related through an unbroken series of DNA replication and cell division (reproductive) events. Based on the information present in DNA together with, molecular level comparisons of how cells are constructed suggests that they are derived from a single common ancestor. This organism, LUCA, lived between ~3.5 to ~4 billion years ago. It implies that each cell in every organism, including all of the cells that make you up, has an uninterrupted history going back billions of years. Exactly how the first cells formed and what they looked like is unknown and unknowable.

The “birth” of a new cell is a continuous process by which one cell becomes two. A new cell is formed when the original surface (or plasma) membrane pinches off to form two distinct cells (→). Note, there is no sharp discontinuity. The new cell does not spring into existence de novo but emerges from the preexisting cell. We often define the start of a "new" life with the completion of cell division. At the organismic level, we mark the origin of a new organism with the fusion of gametes (e.g. egg and sperm cells). There is no sharp discontinuity, both gametes are derived from existing cells and their fusion results in the formation of a new hybrid cell.

The information stored in DNA molecules, an organism’s genotype or genome, is more stable than the organism itself but it is dead. It can survive, for a time, the death of the organism. As we will see, it can be transferred from cell to cell, a process known as horizontal gene transfer. That said, DNA means nothing outside of a system that can interpret the information stored within it.^[50]

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