EVOLUTION FROM PROKARYOTES TO EUKARYOTES Evolution of Eukaryotes.

Prokaryotes.

The first cells were most likely very simple prokaryotic forms. The prokaryotes may have arisen more than 3.5 billion years ago. Eukaryotes are thought to have first appeared about 1.5 billion years ago. There are few hypothesis for evolution of eukaryotes.

Hypothesis.

Definition.

Evolution of eukaryotic membrane bound organelles was related with development of symbiotic relationship between organisms of past. This idea was first proposed by Lynn Margulis. According to this hypothesis, Mitochondria Ingestion. These have evolved when a large anaerobic (living without oxygen) amoeboid prokaryote ingested small aerobic (living with oxygen) bacteria and stabilized them instead of digesting them.

Development.

The aerobic bacteria developed into mitochondria, which are the sites of aerobic respiration and most energy conversion in eukaryotic cells.

Advantages.

The possession of these mitochondria like endosymbionts brought the advantage of aerobic respiration of the host. Flagella These (whip like structures) may have arisen though the ingestion of prokaryotes similar to spiral-shaped bacteria called spirochetes. Chloroplast These have evolved when ingestion of prokaryotes that resembled present-day cyanobacteria could have led to the endosymbiotic development of chloroplasts in plants. Membrane invagination Hypothesis. Another hypothesis for the evolution of eukaryotic cells proposes that  Cell membrane of prokaryotic invaginated (folded inward) to enclose copies of its genetic material EVOLUTION 24 Page | 6 PROF.HAMID NOOR RANA  Invagination resulted in the formation of several double membrane bound entities (organelles) in a single cell.  Entities could then have evolved into the eukaryotic mitochondrion, nucleus, chloroplast etc. Increase in Diversity Whatever the exact mechanism for the evolution of the eukaryotic cell might be, the formation of the eukaryotic cell led to a dramatic increase in the complexity and diversity of life-form on the earth. Evolution of Multicellular organism. At first, these newly formed eukaryotic cells existed only by themselves. Later, however, some probably evolved into multicellular organisms in which various cells became specialized into tissues, which, in turn formed organs for many different functions. Importance. These multicellular forms then adapted themselves to life in a great variety of environments