Figure S1. Origin of the first sex and ways to maintain diploids. Diploids originated from doubling chromosomes during the haploid ancestor’s mitosis without the separation of the cell. In addition to using mitosis, diploid ancestors used meiosis to re-form haploid phages followed by melting the haploid cells to maintain the diploid. Once meiotic products were fused, the first sex, ancestral females, came into being. In the cell in the figure, the red ellipse is a cell membrane, the black circle is a nuclear membrane, blue bodies are chromosomes, and blue dashes between blue bodies are microtubules. The thick red arrow represents diploids evolving from haploids. The pink arrows represent the fusion of meiotic products. The green numbers represent the possible ways to form diploids: ①diploid evolving from haploid, ②two haploid cells with chromatids melting to form a diploid cell, ③doubling chromosomes in a meiotic haploid product without separation of the cell to form the diploid, ④two haploid cells with tetrads melting to form a diploid cell, ⑤meiosis producing diploids when the premeiotic doubling of chromosomes occurs in diploid parents, ⑥mitosis to maintain the diploid, and ⑦parasexual process to maintain the diploid.
Figure S2. Evolutionary steps from the ancestral female to the hermaphrodite.To obtain more mutations to develop genetic diversity, male gametes that might result from more cell splitting times compared to generations of female gametes were gradually formed, and male gamete producers co-existed with female gamete producers in the same individual. A, Fusion occurred between two gametes produced by the same cell, and both gametes provided genetic material and cell plasma. B, Fusion occurred between two female gametes produced by different cells, and both gametes provided genetic material and cell plasma. C, Fusion occurred between a large female gamete and a small female gamete, and both gametes provided genetic material and cell plasma, but the small one provided less cell plasma. D, Fusion occurred between a large female gamete and a small male gamete that generally had a flagellum, and the female gamete provided genetic material and cell plasma; however, the male gamete seemed to actively find the female gametes and primarily provided genetic material in the fusion.