At first, one cell included both gametes; a cell like that is called hermaphroditic, which means both the two genders still reside in the same membrane. They reproduced with hermaphroditic cells breeding with another. But before long, the two genders separated. One gender produced the egg and the other the produced sperm, just as humans do today. Soon the hermaphroditic cells pretty much disappeared and the single-sex cells dominated. Logic suggests a lot of reasons why that should not have happened:
- Males compete with one another for the most desirable female with which to mate. Sometimes these competitions get pretty brutal.
- The process the eukaryote goes through to reproduce is considerably more lengthy in time than is the prokaryotes process. Waste some time, fall behind.
- And finally, the product of the reproductive process includes males and females. Males cannot reproduce. Thus the system is only half as effective as before.
Despite these, the event happened—and had it not happened, Earth would still have no living things but the single-celled prokaryotes.
The rate of evolutionary progress with the appearance of sexual reproduction increased dramatically. Looking backwards, sexual reproduction was a critical step leading to us. Period. Too many argue, “Well, it had to happen or we would not be here!” That statement assumes that the potential benefits are known by natural selection in advance. But the Darwinian process is randomness; pure, unadulterated randomness. Forecasting the future is not part of the process.
So two prokaryotes species game up their identity to make the eukaryotes but the eukaryotes were not gracious all. Their DNA called for ingesting food. To those new, bigger cells, bacteria were food. In fact, after a while a hole in the greedy eukaryotes membrane appeared, allowing the eukaryotes to swim along ingesting the bacteria. That hole eventually became our mouths. But the gobbling eukaryote was still a single cell.
In time, those special-duty single cells began to work together, sort of like “You make the soil, I’ll get the seeds, Al can plant them, and Louise will pick them.” They attached themselves to one another. But they retained their own DNA. Bigger cells had a competitive advantage. The problem: a single cell had a size limit.
Eventually, in order to get bigger, those the cooperating single-celled gave up their independence, in a one sense, to stay alive. They merged together, and became multicellular. The individual direction-centers were merged into one more complex DNA.
Multicellular means that now a lot of single cells, each with special functions, work together. As life becomes more complicated, so does the DNA at each step of the way. The DNA, remember, is the main library where all the directions are kept. As the organism becomes more complex, progress is maintained in the DNA.
Eukaryotes are the new kid on the block. So what, you say? Plants, fungi and animals will evolve from the new kid on the block. “Animals” include YOU!
Fossil evidence of this trip from single-celled to multicellular is scarce. Those first organisms were soft; a billion years or so later they do not leave any marks. However, scientists, working in their labs, have done a pretty good job of recreating this last transition. The evidence indicates the trip from single-celled to multicellular was a step-by-step process, a convergence, and not one big dramatic change.
In the absence of fossils, estimates of when this last step happened conflict. Based on evidence available suggest multicellular life appeared about 1.5 billion years ago, or 2.2 billion years after first life began. However, the appearance of two genders will indeed speed things up.